I'll try to bite on this one, as I have walked the "what cap for a MC30" walk quite extensively.
First of all, I want to explain a basic tenet of this problem, which can be a bit hard to understand fully: YOU WILL NEVER KNOW
EXACTLYWHAT AN ORIGINAL MC30 SOUNDED LIKE. Not EXACTLY.
The reason is that those original caps are 50 years old. You cannot get them brand new, and even if you could, it would be hard to guarantee that they were made EXACTLY the same way the old ones were. Even if you find them NOS, are they REALLY in the same operating state they were 50 years ago? Not likely. And having witnessed the cut open and measurement of some of these caps in MC30s, they are usually not in optimal condition after all these years. Some good working MC30 all original do exist, but those amps have either had very regular use, or extreme care in proper variac (wake-up) techniques after dormancy and stored in dry environments. That's a LOT of "requirements". Most MC30 aren't so lucky to have such well maintained lives. Even if so the caps occasionally fail - they're 50 years old.
So given that (and some opinions shared by my builder) it is my opinion that there is no way to really know exactly what an original MC30 sounded like when it came off the line. We cannot go back in time 50 years to listen to those 100% new off the line amps, and no one has the sonic memory to be able to tell you that they are the same as they were 50 years ago.
Having arrived at this understanding, I set about finding what cap would really be "best". And as I found out, that can be a very subjective thing.....but some generalities can be discussed.
Since the original caps are largely unavailable - and even if they were I wouldn't want to use them at 400+VDC - you must choose something else, that is if you want them to WORK. If one is a collector and wants his all original MC30s to set on a shelf, you can put originals in there. But NOT for actual USE. People with good, all original capped MC30s are RARE - those caps usually fail in time.
I asked this exact question re: caps several years ago, and (at least in public on forums) didn't get much specific info. I gather some of this was due to the subjectivity: If I recommend an ampful of Sonicap Platinums, and you buy them and install them, I might not want the $600 plus other parts and labor bill hanging over my proverbial head if you do not like them.
Also, there have been much more online experiences shared since then, and many cap brands and options have surfaced. When I was asking, "amplifier cap rolling" wasn't as much of an online sport, and generally speaking the online communities have tried and shared results which give us a better idea of some choices available now vs. say, five years ago.
I was fortunate to have a fellow forum member here extend a hand and offer to have me sample some builds. He sent me several examples of MC30 using different parts packages. We used Illinois Capacitor caps, Sonicap white, and Sonicap Platinums in that initial series, and it was enough to come to some basic understandings.
The general signature was basically the same: The full and rounded bass response, and the 6L6 midrange was intact with all of them. But the
details are where the changes occur from cap to cap. As a Heritage Klipsch speaker owner, the amps do reveal subtle nuances and color differences from cap to cap, as well as from tube rolling.
The Illinois were sufficient for most builds, and a good build with them showed just how much was missing and different from my original builds. The congested throat and syrupy nature that became evident from the original builds was gone, and the bass response was MUCH better defined. OK, I now know I'm on the right track, and for most users would be very happy with it. But I'm anal and want the absolute perfect "finish" on my uber-revealing horns, and the Illinois did exhibit some edginess with my preferred tube combos - the extra details might even be a benefit on other speakers.
We tried a few more cap combos....then got sent a pair of 30's with two sets of different parts. One with Sonicap whites (standard good quality film and foil) and Platinums (a top-flight, uber revealing, audiophile cap that costs 600+ in parts per set of MC30 to do). Our thinking at that point was "OK, we know the benefits of a good base build - what if we go "better and best" on the caps and see what we end up with? When I got the amps from our builder, the deal was to see how long it would take to see which amp was which. It took 30 seconds. I listened for a whole day just to say I did it.
Then a second amp was built to match the Platinum version, and burnt in for over 500 hours (necessary for teflon formulated caps of any kind). Once at the 1000 hour point, a pair of amps revealed themselves to have all the general "body" of the MC30....but the
details......WOWWWWWEEEE:yes: An almost SET like detail, a blackness of background that really set the build apart, and a bullet-like accuracy and speed in the microdetails. On premium recordings, it's OMIGODITSOUNDSSOGOOD, on some of the imperfect recordings, it's a bit too much detail. I considered it a great success, though, this amp pair being my "SET with testicles" in my stable of amplifiers. These caps just seem to get out of the way 100%, showing just how good that circuit and transformer can sound. But I do recognize that this is not what most picture an MC30 to sound like, at least with respect to those details.
That "platinum" build was so good in what it was good at that I dare not touch it. I acquired a second set of consecutively numbered 30's during that process, as during these trials I was becoming aware that I was such a fanboy of the amp that one build wasn't gonna be good enough. I thought the extreme openness of the platinums was a bit different than the "classic mental picture" enthusiasts have for an MC30. Just a bit TOO "audiophile". So we're looking toward the more traditional expectation of the MC30 - what recording engineer Steve Hoffman uses to bring those older recordings back to life.
My thinking had me leaning "PIO", at least in terms of a general sonic signature of a butter smooth kind of sound that give those older recordings a chance. Granted, I could use a more typical film and foil and get something "in the middle", and might well do that if I only had one set that would work well all around on my horns. I was recommended a particular cap for the purpose: the K40y-9 russian military capacitor (commonly available on that auction site). Given the reasonable cost, I figured I could afford to try them....it wasn't as if I spent $600 on the caps
Once run for 300 hours they settled into a nice, easy, warm sound without losing TOO much detail. Vs. a really "fast" cap, they aren't as detailed, but in the overall, they are reasonably well detailed. And for those older recordings that need a little help....really nice, lively, and smooth. When I go to my BEST recordings, I can tell the difference in the more settled down edge definitions, but that's not what I built them for. Yet I could play most recordings and have ya toe tapping and loving every minute of it with that classic Mac tube sound. The K40 is a REALLY safe "bet" in MC30s, and they aren't boutique priced.
This second pair became my "Steve Hoffman" purposed MC30s.
NOW, you can also play around a bit and get something "in the middle". For example, I could take the K40 set and roll an interstage cap to a film and foil, or I could take the Sonicap Platinum build and roll in a K40 or two. OR, I could maybe use an audiophile quality film and foil across the whole build, like an Auricap or Hovland or even a white Sonicap and get something "in the middle" of my two builds, If you are a do it yourselfer you can really have fun with this if you want to "get into caps".
There are a lot of caps in the audio path of an MC30, and they do impart some "signature", even though the general nature of the sound is similar. It's the details that count. If you have horns, or are otherwise similarly anal as I am about these things, you might care about the things I have typed here. Similar advice on the resistors: If you're anal, use carbon film or carbon comp resistors for replacements (and have your resistors checked). Carbon film are more stable, carbon comp are ideal sonically but can drift in value with age and humidity.
As for look: You aren't going to likely find NOS bumblebee caps that will work at those high voltages. If you plan to use them, there is no way around this: ANY OTHER cap change will deem them "unoriginal" no matter what they look like - and anyone who cares will know the difference in ANY other branded cap that's in there. The other "approach" - what I did - was for SOUND. And USE. A GOOD build will still hold good value. So, it's like a classic car: Do I take it out of any stage of 100% originality in order to make it driveable, if that's what I have to do to make it driveable? If you are at any stage out of originality now, your amp is NOT original, so you might as well get a good after-build with proper parts for the purpose and be done with it, that is if you want to USE them.
Just want to give something best i can afford to my beloved amps. Have you ever tried the Nichicon, or have any idea?