You guys are where I was at a couple of years ago after I bought my Cornwall Is. I decided to skip dipping my toe in tubes and just went for it with a Jolida Fusion 3502P...which is a great amp, albeit push-pull. That Tubecube7 has me thinking about SET again.
Here's what I've learned:
When it comes to 12ax7/ECC83 tubes, NOS is king. SOOOO much better than anything being made today. I hate to say it, bmoran, but those Electro Harmonix just scratch the surface. It's both been an adventure and a curse searching for the best sound.
When it comes to power tubes, I've found the best sounding current issue tubes are all made by Gold Lion. I've tried everything else, except NOS, which are prohibitively expensive and have a minimal impact on sound compared to your 12AX7s.
What's worked for me, in MY system, with MY preamp, and MY amp, and MY Cornwalls, in MY room, with MY ears (because all of those come into play when it comes to sound quality), is a set of Gold Lion KT88s with either Tungsram or Matsushita 12AX7s...and I'm still testing to find the best set of 12AT7s in that combo.
It's wonderful, it's exhausting, it's obsessive, it's rewarding, it's cool, it's amazing, it's a gigantic time suck...but welcome to the world of tube rolling.