Tube favorites for Modwright LS100?

For the 6SN7 I'd give the Shuguang CV181 Black Treasure tube a shot. These take a bit of break in time and in my experience will fluctuate widely in performance in the first few hundred hours, but the stable performance after that will be worth the patience. I do not have experience in that preamp though. You might be able to find a used low-hours pair for $150 or so. There are some NOS Russian variants that are quite good too, but their prices seem to be going up dramatically lately...I think the pair I have that I really liked is a metal-base 6H8C with round-hole plates. The plastic base version of the same tube is also pretty good and cheaper. As far as the rectifier tube, a sleeper tube in that family is the GE copperplate. You might also try a 5U4G, like an NOS Tung Sol in there as I think you can use that tube family as well.

I preferred the Black Treasure tube in my MWTP over the very dear NOS TungSol round plates. I also prefer the Black Treasure in a headphone amp I'm testing out right now, compared to the same TSR. Go figure. I did not like the Black Treasure tubes right out of the gate though. I think you'll find there are bargains to be had from those who don't give them enough time to burn in and just sell them out of frustration. I have not tried any of these tubes on the LS100 you own though so take this input with that caveat.