When I had those amps I used EH KT88 tubes. A KT88 will give you a bigger brighter sound than a EL 34 type tube. It all depends on your taste!
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I love my Mid Monos... KT66 sound great , as do KT88. I am running Mullard repops in them now and they sound great too.... so when I need a change of pace I will swap types . Overall I think I prefer Gold Lion KT66 but the quad I had was getting tired so down the road I will get another quad. I would love to hear from anyone that has used the new 7581a tubes in these amps...
I'm a fan of GL KT88s and have used them for years here and there. However, the Gold Lion KT77s, at least in my single ended amp, are amazingly better sounding than any other tubes I've used in this amp, including KT120s, 88s, and 150s...I was surprised by this as I had tried a pair of JJ KT77s and they sounded not so good, but they're a completely different tube...now on my second pair of the GL KT77s, and man...a great tube.
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.