Wolf--Pretty sure your "raving," at least subconsciously, influenced my decision to pull the trigger on it. I bought my amp lightly used. It came with GL KT88s, a vintage RCA 5u4g, and a metal base 6sn7w(!). I've read that Mr. Had pretty much built these to be able to take just about any tube that will fit. But, I'm afraid to just start changing without knowing. I'm pretty sure KT77, 6l6, 6ca7, 6550c, kt90, el34, kt90, kt99, kt66, kt120, and kt150 work, but not sure if the rectifier needs to change with the power tubes. The kt150 is supposed to kick up an extra watt. It's the one that's supposed to do 17 watts with KT88 and 18 with kt150 (with the big square output transformers). Can't imagine the tubes last very long burning that hotly, so need to at least have some spares ready. It sounds great as is, and I imagine the prior owner took some time to roll to the specific tubes. No "need" to change, but sure wanna try it.
I have a bunch of 6sn7s, but no Amperex. Maybe I'll hunt one down.
Just for fun, I hooked it up to a pair of power hungry Salks, and it actually sounded good--at conversational level. It lost its mojo at a reasonable listening level. It's the best sounding amp that I've tried with Omega 3 HO, even better than with much more expensive PSETs. It also sounded really good with Sophia Princess Model 2 (88db). It's a small carpeted room, for now. Basically taking over the guest room with gear and disrupting domestic tranquility.
John: How do you like your SLP98? I have a red one and really like it, but it does this weird thing--as I turn it up to rockin' levels, the singer gets larger and larger--not that the singer gets closer or louder, just larger and takes up more space. Weird. Swapping things in and out, I've narrowed it to the SLP98. That's my only criticism of it.