I got mine from that auction site years ago, when I was collecting tubes like the Cookie Monster collects chocolate chips. Got tipped off by a local tube tech who told me what to look for, and found a number of them here and there - had the noisy ones screened out and ended up with about 10ish suitable for line stage. 6U8 can be a finicky tube, if they settle in for 100 hours they usually stay that way for a long time. burnt through a few to find a given pair, I probably had 25 total and ended up with 10ish really good ones. You buy 6U8s by the batch as they can be flaky for line stage use. Don't spend big bux on Teles or some "dream pair" unless you like to gamble. I think I spent 10-15ish per tube and that was later in the process when tubes were really getting expensive.
I would check the "radio" types before the hi-fi types on tube sellers though. Sometimes you get lucky and find some of those radio parts tube sellers who will have a stock of something like this. They don't prescreen usually for a tube like this but sell them NOS, you get a screener to weed 'em out for you. I've saved thousands on bulk type stuff from sellers like this, who sell primarily to people who have their own testers and price the stuff accordingly. I don't have that equipment, but have a local tube pro who does, who screened all of my stock worthy of the effort and weeded out all of the duds (which were few and far between).
I think the 6U8 cost the MX110 a good reputation for a lot of years, until people really found out what they were missing. Some might have had issues with the 6U8, or didn't like the particular 6U8 installed (I don't think RCAs are all that in this piece) or had flaky/noisy versions that the owner didn't want to keep messing with to find that good set - or refused to even consider it because it had a "pentode" for the love of jah - in the linestage of all places. Just not "audio kosher". But chosen properly, it works really well, and if it sounds good the "theory" as it were pretty much goes out the window.
It's not as big of a deal as it sounds, but I tell MX110 owners to have 6U8 by the sleeve if you intend to own that pre for "the duration". The Tele 12AX7s will last you until you are dead in that pre (they run like 110 VDC or some low number on a bunch of those slots IIRC - a day in the park for Teles), but the 6U8 you want a backup stash for. And good ones at that, because they do vary in sound. Now I won't tell people NOT to buy Tele 6U8 but they aren't the end all be all either, especially for what some charge for those.
