My amp is a VTL, not an ARC. I tried the new Tung-Sol 6550 and didn't care for it in this amp (same story with the SED KT88). I like both the SED 6550C and the Electro-Harmonix KT88 better than the others I tried. The latter is my own long-term choice, though the former is a warmer-sounding tube, and to some may better complement the sound of an ARC. However, I believe ARC does use EH power tubes as original equipment -- I haven't heard the EH 6550, which might be the stock tube in your amp. In any case, there is no difficulty identifying or obtaining the SED 6550C (St. Petersburg factory) as opposed to the new Svetlana (Reflektor factory, same as Sovtek/EH) -- only the SED has the Winged-C logo printed on the envelope, and most tube-supply houses carry it. If you have an indication that the tubes in your amp may have been used for 3 years or longer, then you should go ahead and retube whatever your choice is.