The loudest bass I've ever heard from a single unit was my JVC MX-G50 with dual 6.5" subwoofers playing in stereo.
It makes me laugh mate, most of us are obsessed with bass lol, it's better than sex, as for best bass i wouldn't know where to start but as for boomboxes the jvc rd md5 is a good place to start bud
I have two of these little panasonics lol (with the Line in on top) Sound is very good, with the xbass, good for picnic or camping... (when you don't want to expose your best boxes to the elements) Our family had one 25 years ago and I used the Line in a lot. When I saw one in the wild for peanuts, I bought it. I should switch the speakers and all with a bad sounding but beautiful box...
Here are the 3 most underrated boxes in my opinion: GE 3-6035A, Panasonic RX-5050, and the Sanyo M9982 (a.k.a. Graetz GT-460). These all make very good sound but don't cost a lot.