I've got my Jet City back! I haven't had a ton of time to play through it, but here are my observations thus far:
1) The amph is quieter--considerably less noise.
2) I may have a pinch more clean headroom. That was unexpected. I'm going to test that more.
3) The full range of the preamp gain knob is now usable. It gets a bit wooly at 9 (the top end of the range), but previously it became a bit of an indistinct mess. Even at 9, the noise isn't mind-bending.
4) The presence knob is now usable. I can turn it up a ways and not enter "rabid bees in a tin can" territory. I still would never turn it up past six or so, at least not in my office. My guitar is pretty bright, so it might be more usable with a very dark guitar.
5) I can see the tube glow better through the metal baffle, so it looks cooler. That has to count for something, doesn't it?

It turned out the way I hoped--it's still my amph, but moreso. I kept the Chinese tubes in case I'd blow one or if I want to swap around preamp tubes or something. I'll try to post some clips soonish here.