So, closing the circle on this... I've been inside the amp for the HT12ps and saw NO evidence of failing caps whatsoever. In fact, it is working fine... problem turned out to be horribly soldered lead wires at the actual woofer inside the box, making the woofer intermittent (mostly off). They were so bad I can't imagine how it ever played, but it did! For years in fact. They were captive inside the terminal posts but only because the cold solder blobs on the ends were too big to slip back out of the little cylinders the leads route thru where they emerge towards the cone. I put a bunch of heat on the terminal sleeve and flowed the joints properly, should hold up forever now.
Also found out (re-learned might be a better description) that my HT12ps is a total prototype. I remember vaguely rescuing it from being thrown away, and that the ports weren't 100% finished internally but after taking the cabinet apart to remove & service the woofer I really see the non-production details now - including a bolt that was supposed to be holding the woofer in (one of eight) that was hot-glued in place! Apparently the captive nut came loose from the MDF and rather than pull the woofer to service that (the captive nuts install from inside the smaller sealed chamber at the top of the woofer, so the only way to get to them is with the woofer completely removed) someone just hot-glued the bolt head so it wouldn't rattle (which worked, amazingly). I fixed that, also fixed the damaged thread that made it bind up and come out in the first place, and resoldered the leads correctly.
The woofer in this one is actually not production either, it has a big old Ground Zero driver in it - looks like one we might have had made locally during the development project. Damn thing vibrates the whole house again now that it's fixed though - time to go on a hunt for more drywall screw heads popping out. Definitely run circles around the HT10ps I had in there temporarily, and that one is no slouch either.
John