Have had best SQ using 807 tubes.
Tried EL34 and did not like any. I have a bunch of 6L6, 6V6, and 6P3S-E / 5881 / 6L6GC. I am very curious if these would work, it’s an excellent tube and possibly hidden gem in its performance based on how good it works on different amp I have that’s actually specked for it…

Currently running KT66 and all seems fine along with great sound quality and character.

A friend of mine let me check out his McIntosh MC60, which he had recently purchased on ebay. He's a rookie in stereo equipment, and when I told him that he had bought a mono-block, he was a bit disappointed that he would have to find another one for a full stereo setup.  But I told him I would be glad to check it out, to make sure everything worked.

This amp came stock with it's original tubes, KT88s and I believe they are original General Electric...couldn't really tell though as the labels were rubbed away, or what was a sticker was gone. But they were not reproduction tubes.  In fact all the tubes, were original.  Telefunken 12AX7s, GE 5U4G ...just amazing.  Nevertheless, I thought it would be fun to try the KT88s on the Rachel, to see if they had life in them and what the sound would be.

At first it took a little bit of time to let them come up to current, but they made it to 40mA, except for one tube seemed to have a bit of an issue.  Some gentle taps and movement in the base seemed to do the trick and she was up to 40mA.

I then put on some music, XTC's Ball and Chain 45rmp 12", and what I was treated to was a very detailed high end, and lows...well it seemed somewhat revealing.  I could hear the skin and air of the kick drum and it's resonance, was one of my initial impressions.  The highs were silky...I was just in awe of what I was hearing.   Non-fatiguing sound as I just didn't want to turn off the system ( had to as I had "responsibilities").  Everything I listened to, from CDs to vinyl just seemed to really come to life.

This tube replaced a pair of Seimens GrooveTube EL34s that I've had since the late 80s that were originally used on my Marshall amp. But that's one set of the many tubes I've put into the Rachel.   I have tried 807s, 6L6s, KT77s and various brands of EL34s.  All have their own characteristics and quite good and pleasing, but these KT88s made me hear something different than those other tubes.

All I got to say is that I really don't want to give these tubes back to my friend, but will have to soon...but in the meantime, these KT-88s seem to be my favorites.
I'm looking at prices for replacements.  Originals are totally out of my price range at this time, so I see there's Gold Lions out there, which I think I could swing...but these are not NOS so I'm not really sure if I should take the plunge.