Tubes to replace stock in Quad II Forty's amps?

Don't worry about the source of your woes. The tubes are frequently the source of snap crackle and pop then they just stop unless you haveno fuses in which case a major tube failure will cause all manner of havoc in that case don't wait. Yes input tubes are the slowest to go but they do die on rare occasions, I even bothered to see if there were good subs in case you couldn't find those. There are but they seem obscure.
The output tubes are a matter of preference, but I have heard the KT-88 of choice these days is a GEC repro from Penta labs Chinese yes Penta are considered tops these days. I don't use them in my amps ony because I use EL34L outputs, Also if your amp can take KT-88s then it should tolerate 6550. I understand from my tube preoccupation that the Tung Sol re-issue is a good one. Do yourself a favor and do a search in AA's tube asylum section for both.

SOVTEK makes very good KT88 tubes & do not forget GROOVE TUBEs KT88s.

The 6SH7 input tubes are supposed to be millitary grade! Gha! Get something with a glass envolope like TUNGSTRAM or ELECTRO HARMONIX.

You do get better 5U4G rectrifiers also from SOVTEK & GROOVE TUBE. Do check out GROOVE TUBE - they hand-match their tubes to very fine tolerances.

Kind Regards,
Dewald Visser

Will an input tube 6SH7 in this case, take out anything if it goes bad or is bad in an amp? I am worried and hope to get some NOS ones off ebay.One seller has untested NOS military 6SH7GT. If these are bad will they destroy some other component down line in my already troubled amps? Thanks

Avoid untested tube unless you can test them before you put it in your amps! A faulty input-tube can destroy a power-tube... Be very careful!

Regards,
Dewald Visser