Thanks for all the info! It really helps make sense of everything you've tried and what you had to do, and still do, in order to make it all happen.
Wow, nice job on the upgrades! I bet your unit sounds so much better than a stock one that you'd have trouble telling that they are the same amp without seeing them. I may look into upgrading the pot at some point and it's nice to know that the TDK fits perfectly, while the Blue Alps mhamel I believe installed needed a little tweaking.
I see what you mean about the Cossors, I guess I figured them to be ceramic sense the ATS25's are and the only pictures I've seen, they had white bases as well. Very cool that yours have brown instead of white, I had not seen that before. I have several ATS25's, one pair is even hand-painted and no other stamping since it is so old! They are very good and their plate structure looks quite different to a normal 807. I tried a very nice pair of NOS Mullard QV05-25, but these do not work with the amp as is and they blew my FET, so I do not recommend them unless you think you can tweak the amp to handle them. Other than that, I have all 3 versions (different getters) of the newer militarized 807s, the 5933, and they are pretty good. I've always felt that they were a bit more solid-state sounding than the general 807s and lack some of the tubey sound in general, maybe even a bit clinical in comparison, but they are worth a shot, if at least to see how they respond in your system.
I do have a pair of Tung-sol 6550's and they do sound very nice. I also have a pair of RCA's and GE's, both the coke bottle and the ones that look like larger 6L6GCs. To me, the GE 6550s that look like larger 6L6GC's, with green lettering I believe, sound the best, then probably the Tung-sol and RCA's. The GE coke bottle ones were my least favorite. I also have not tried an original Genalex KT88, as they are impossible to find and if you do, they are way too expensive. I have other KT88's though (Penta Labs, Mcintosh, etc.) and to me the 6550 shares many characteristics with it; they both have great speed and slam. However, I think I generally prefer the more musical and open sound of the KT66, with my smoked glass GEC pair being my current fav.
As far as EL34's, I've found some to be very nice, while others have fallen a bit flat. I believe others have had better results with EL34's in UL mode rather than Triode. Some of my favorites have been the Mullards, Siemens, Telefunkens, and Tesla's (both yellow label and "blue" glass). One that really surprised me and is also one of my favs is the RCA Japan later production, which I got for cheap! I have also tried the RFT's as well as the Winged "C" and they are both nice as well, giving some of the best soundstage, as also with the Tesla's. Many of my pairs have had issues with the pins not being properly tapered off, so I use socket adapters for them as to not damage/ware the amp's sockets when I roll. In general though, some of the other power types give me more impact and weight in the low end as well as speed/attack than the EL34's due in general, so I only really use them for when I'm listening to something that I want to have a more delicate/intimate feel to, such as classical music.
I'll let you know if I have any questions before I try some 6SN7s, etc. It may be a while though, as I only really have singles that I've used with other amps and would need to get pairs. I've tried a ton of 6SN7s in the past and it is a very fun tube! In my opinion the Tung-sol Round Plates weren't worth all the hype, as others gave me more of what I was looking for and at cheaper prices. Same thing went for the "Bad Boys" as well as the RCA smoked glasses; both fell a little flat in my system compared to others. I tend to prefer anything militarized, especially the brown bases and also "T" plates with square or "D" bottom getters, such as early Tung-sol and CBS. The Brazilian aluminum RCA's also really kick the sound out as far as soundstage goes, the Brimars are quite good in the highs, the black glass National Unions have a ton of bass slam and extension, and my RCA smoked glass military brown base gives great sound from head to toe, if not for them being pretty rare.
As far as 12au7s, I agree and tend to prefer the variants a bit more than the standards. My favorite currently are Amperex 7316s, 5814a (both CBS and Westinghouse a bit more than the RCA's, etc.), my gold pin 7730's, CV4003's, Brimar 13D5's, and Tesla ECC802S. I also tend to like most of the European types, except for a few Russian, ECC82's generally more than the standard 12au7s.
It all just really shows how crazy tube rolling can get, especially with this amplifier! With so many options you have to be careful not to drive yourself nuts! I'm just glad others have also had such great experiences with this amplifier and its versatility. For me, it is the amplifier I will hold onto for the rest of its days, enjoying every ounce of musical bliss it extends my way. It also all reassures me that my review and constant recommendation of this amplifier is not only true to my own experiences, but to others as well.
Happy listening and keep letting us all know where the journey is taking you!