I have ARC VT130 (which use basically the same tubes as all the VT100's) and I have tried both the KT88's and the KT90's and always go back the 6550c's. Tighter bass then with either. The manufacture is Svetlana. Only buy the 6550's that have the "C" after the number. There are several 6550 tubes, again only with the "C" after the number. As stated in one of the above threads buying from ARC can get expensive, either buy from The Tube Store or I have been buying from a company in Chicago called Triode Electronics, they are an internet company, very good company, they took back some tubes I had had for over 6 months just because the tubes would not bias up in my amp, recommend them highly, good luck.