Pretty sure it was Hitachi OEM; should be more than decent. Construction quality was a bit subpar compared to similar integrated amps of its era (e.g., Masonite back panel, as I recall).
I've seen the SA-1000, for the size it's pretty hefty, I think the masonite panel is just a covering for decoration. Never heard it in action but it has 3 tone controls, might be pretty decent.
I had two SA 1000's in 1975 running 4 speakers and the matching tuner. They were nice and 25 wpc was not bad then. The tuner was the gem. I also had the matching 8 track recorder.
Thanks dswankey, I think there is a difference of 5wpc in the 20wpc sa1000 and the 25wpc sa1000a, so I will keep looking. I emailed RadioShack in hopes they keep an archive.