No disagreement here, at all. Just wondering your reasoning, sonics, price... fitment?
There's a few reasons, but the main one is because of the properties of capacitors. Specifically, ESR (equivalent series resistance). On average, electrolytics exhibit around 2-3 ohms of ESR, less if they are of the low loss variety. Mylar film or polypropylene capacitors, on the other hand, have almost no ESR at all. This resistance affects the balance of the speaker, and when designers create crossovers for two, three, four or more way speakers, this comes into account.
Essentially if you take an electrolytic and swap in a polypropylene (for example, those inexpensive Dayton audio polypropylene capacitors) it is like removing a 2 ohm resistor out of the circuit. I'm sure as you can guess, this negatively affects the balance between the drivers at that handoff point. To further complicate this, ESR is like the impedance of a speaker in that the resistance varies over the frequency range. This is why I mentioned the fact that Mundorf M-lytic capacitors have similar ESR properties to Alcaps - if you choose an electrolytic that has somewhat different ESR properties, again it could change the balance of the speaker. This is why people comment that polypropylene capacitors sound "bright". In fact, they probably don't, it's the relative difference in the change of balance to the speaker that makes people perceive them as bright.
Now of course, you can go ahead and swap it with something different, but in high quality designs (like the one that is the subject of this thread), generally there is a lot of thought and time that goes into the crossovers, so you will more likely achieve the best results by simply replacing the parts with parts that have similar properties. This is why I stress to replace Elcaps, which are popularly used by British speaker manufacturers in the 70s and 80s, with Alcaps, because they are very similar and will retain the original balance of the crossover. Andrew Jones, whom we are all very familiar with, in fact said the very same thing, that when recapping you need to be aware of the ESR properties of the original capacitors because designers factor that into the crossover.
Anyway, I'm not sitting here saying it is wrong to tinker or tweak. That's part of the fun. But my philosophy is that before you do that, at least have a chance to hear the original design in a restored state so that one, you can appreciate the original design, two, so you have a baseline and three, it may end up sounding so damn good that you won't bother going any further and save yourself the money and hassle. I went through a bout of modifying a pair of Celestion 66s where I used polypropylene capacitors, and I eventually went back to electrolytics because with the polypropylene capacitors the speakers sounded unbalanced and wrong. They were in fact unlistenable it was so bad. I even added resistors in series with the capacitors to compensate for the change in ESR, but what I learned when I measured them is that all it did was lower the SPL at the crossover point and didn't really solve the problem. I've since attempted to do the same thing again, and the same thing happened - the original design using like for like parts sounded best. Since then (and even before then) I have always replaced like for like, and the results have always been very rewarding.
That's why I suggest to replace electrolytics with electrolytics.
If something I said doesn't make sense or you would like me to elaborate more let me know.
