I just put a new set of preamp and power tubes in my 5:50. 3 JJs(V2,V3,V5), a GE 5751(V1) and a Sovtek LPS(V4) for preamp, and matched Tungsols for power.
Now the amp can still be heard, even when the volume is turned ALL the way down on both channels. Not loud @ all, but definitely audible...