kt66s not exactly a drop in for 6l6s, they have a different plate impedance and this will change your output tap secondary impedances.
I have used JJ Kt66s before, they have a big glass envelope as large as their Kt88s so space might be needed if your amp is cramped. they bias up the same tho...
That guy is kind of back burner for the moment while I build a ton of custom shit but maybe someday
This is the current amp I am wiring up, its a 200W 4 x KT88 guitar amp super clean single channel doom/drone crusher. So yes it can do some Sunn0))))...