These amps have been a popular and relatively inexpensive way to obtain a tube amp with moderate power. The design dates back to the 1960's and there were many of these produced. However, IMHO the sound is the old school tube sound, which means rolled off in the high frequencies and very "warm" sounding. These amps don't have nearly the collector interest of, say, the old McIntosh or Citation tube units. I have a couple of refurbished ST70's of my own sitting in storage.
If your units have been sitting in an attic for years, subjected to extreme heat and cold, the passive components (resistors, capacitors, switches, etc.), which were never high quality to begin with, have almost certainly deteriorated to junk status. That means the only real value might be the chassis and the transformers, which is a fair starting point for a total rebuild. That won't be hugely expensive for somebody who is willing to do the work.
