The new Mid Monos apparently have separate bias for each tube and can take KT120 and KT150.... i asked Mike from Quicksilver a while back if KT90 were ok and he recommended against using them... He said that they were voiced with EL34 and he recommended EL34 . When I bought my Mids, they had big bottle 6l6 and they were ok, but I didn't know how many hours were on them so I installed EH EL34 , the ones with the green label. They actually sounded really good.
So if you have the older style with single bias pot and L.E.D. I would call or email QS to see what they caution agai st using.
So if you have the older style with single bias pot and L.E.D. I would call or email QS to see what they caution agai st using.