agranttz,
Update: today I swapped back and forth between my very best vintage 1960s Mullard 12AU7s and latest version PSVANE MKIIs in the QS Mono 120s. My buddy did the same in his Linestage last week, replaced the stock low grade JJs you have now. They do sound lean in our rigs too. Both of us gave the lower version JJs away. There are much better JJs up the line into the high $ Black Sable JJ versions but those are more costly than several vintage and 40% more cost than the new MKII PSVANEs which are decent for the $ if you don’t wanna mess with vintage.
Personally, I’ve tried the best from vintage 1960s Mullard, Tungsram, RCA, Sylvania, CIFT, Brimar, Amperex and more in my QS amps. Tried 17 different vintage pairs (some on loan) and 3 different new-reissues pairs in total. Same with my friend with the QS Linestage and QS Mid Monos. Both of us agreed, all so much better than the new low version JJs that came with your QS Linestage new-stock from Quicksilver. And, gotta say the new PSVANE MKII 12A?7 tubes are pretty nice too. Maybe a little fuller upper and lower midrange, maybe a tad more forward than my best Mullard "Blackburn" small signal tubes, but not bad ad all. With more burn-in they are settling back a tad more now. I like both, a lot. The Mullard vintage and the newer PSVANE are my #1a and #1b for different reasons. Have some of the latest "12AT7" PSVANE MKIIs coming this week to re-test those again too.
The QS Linestage and Mono amps are VERY responsive to replacement of quality small signal/driver tube changes, fwiw. And, with really great interconnects we noticed an even bigger difference with changes in the small signal and driver tubes. Hope this helps.
