I'm sure someone will chime in here with the exact model/details, but around 1975/76 I believe, Bob was using a Mark of some sort (as was Jerry at times, too). Blues for Allah album was recorded at least in part using Marks (again I think).
Based on the dates, I suspect those amps would have had to have been Mark 1s, as the II didn't come out until 1980. Also: in the later 80s (or maybe 90s) Bob used a Mark IV for awhile. I would think that almost any Mark variation from a Mark I through Mark III could work very well for you and be found in your price range (the Mark IIC+, however, is a cult amp and will be way more expensive--let the Metallica fans have 'em). The Mark IV is a possibility, though it's a bit more modern sounding to my ear and may be harder to find a decent one for $800. Given your "post-hiatus" tone comment, I'd say one of the others might line up better, tone-wise.
There are total fanatics--maybe even some here--who may have much more detail on these amps and be in a position to suggest a very specific variation (i.e. Mark IIa vs. IIB vs. IIC. The Mark IIIs came in various iterations, denoted by a colored marker "stripe" applied next to the power cord. Some are more aggressive than others.