Ekco EV55 using 6550's in place of KT88

I purchased an EKCO EV55SE from a fellow Audiogon member and since then I've been trying to find out if I can substitute NOS GE 6550's for the KT88's ? I swapped the EKCO labeled KT88's for a quad of Gold Lions but really want to try the GE's also. The previous owner has been a tremendous help going way beyond anything I could ask for. I've contacted IAG via email on a couple of occasions and even called a dealer in the UK but can't seem to get an answer. The previous owner suggested contacting  USA Tube audio as they are a supposed dealer. Well, that was a lesson in futility. No help they just trashed the amp and tried to steer me to another product line they carry. I believe the swap would not be an issue but would like to get some verification before I swap. Don't want to hurt the amp nor lose a tube or two as a result. Thanks.