Well at least the one shown popped open has been recapped to a certain extent. They look fine. You can never know the condition of the pots, but they used good ones. precision wire wound resisters for low ohm apps. I took out the stereo feature and hard wired the little jumper thing going on. One of mine has developed a hum in a filter cap that I need to change.
Closest in sound, major US company that built tube amps would have been the 7189 PPs made by HH Scott, if you want a sound comparision.
Calrad, Monarch and Coral all came from the same factories I think. Calrad was always good. Monarch was not always as good as Carad.
6BM8 is a fine tube, I have found the RCAs to be very good, Slovak tubes labelled Westinghouse are good. Mazda is supposed to be good. Might be predudice but the old Japanese tubes have never been my favorites in the 6BM8 and I usually change them.