Tube Rolling ARC VTM 120

In my ARC VT-60 I use EH 6922 and SED 6550 tubes; it is a wonderful combination! I think that your VTM 120 amps are the mono version of the VT-60, if so I don't think that you will be satisfied with the EH 6550 tubes.
Craig

What is the "SED" ?

The best 6922 replacement you can get is a 1960's Siemens CCa. You can tell it is the right one because it has 2 or 4 ribs on the top, and there are no silver supports under the plates. Telefunken CCa's may also be good, but I haven't personally heard them.
The best replacement for the 6550's is probably MOV/GEC KT88's. The Chinese Shuguang Gold Lion KT88 copies may also deliver a substantial amount of the performance, for a tiny fraction of the price. I have tried several different tubes in my Ref 300 Mk II's (SED CryoValves, GE 6550A's, British and Chinese Gold Lions), and I have finally settled on 1/2 British Gold Lions and 1/2 Chinese Gold Lions. If I had to pick between the two, I'd probably pick the Chinese. The SED's have EXCELLENT bass when warmed up, but the midrange and highs are not up to par with the Gold Lions.