so i am no veteran perhaps but here is my 2 cents.. i have electro harmonix. 6L6 and rca 6sj7 pre's. the black metal ones. the sound is flawless. no hum at all altho this may be true of any tube in this amp. the only upgrade i see is the kt 77 genalex offered on e bay for 40 a piece. from all reviews i have seen this is the best advice i can give you. fwiw, this is an AMAZING amp for the money.