Thank you, N9Voc! I had forgotten I'd asked the question so now I feel like a loser for not replying sooner. I had tracked down some information about the amp because there was a complete version on ebay about a month ago. I assumed (bad move I know) that the amp could drive 8 ohms with no trouble and built a speaker cab that reflected that fact. The cab is based on the old Silvertone 2x12 and is loaded with two old (1960s) Quam Alnico 8 ohm speakers.  I rigged the cab so it could be run in series and parallel intending to run the Sears amp just in series (16 ohms), of course I accidentally ran it in parallel (4 ohms) and fortunately it worked fine. Just for once Sod's Law worked to my advantage! So it looks like a Sears 60bxl (or this one at least) was designed to drive 4 ohms. I also built a output (series or parallel optional) into the cab and the amp drives a third 8 ohm speaker fine as well, it sounds sweet if I do say so myself. Thanks once again.

I recently picked up the chassis unit from a Sears 60 BXL and was wondering if anyone knew what this amp was rated for? Or maybe there is some way of calculating this that I am unaware of. (I'm a total amateur so this wouldn't be surprising.) I'm planning on building a cab to go with the chassis but for obvious reasons would rather not fry the amp. Any help would be greatly appreciated.