I'm going with the Beanie Babies, they are so cheap right now, even with the tags on them, but you know in a few years they will also be worth a fortune! Unfortunately the golden days of hunting vintage audio are starting to move past us.
The serious collectors are always looking and asking. They sold millions of boomboxes, Walkmans and audio equipment worldwide and a good portion of it is still out there buried under the dust of the basement, it just takes a little more work. The guys finding the good stuff are hitting garage sales, thrift stores, estate sales, and looking online all of the time. I've found stuff at the vacation back-road garage sales and the crappy Salvation Army Thrift Store with nothing for weeks but then a Sansui Amp pops up out of the blue.
Always be looking and always ask, I snagged a nice boombox last winter for 20 bills at an antique store after asking, it was buried behind some Nancy Drew Books.
A lot of the 70's boxes with analog VU meters didn't (and still don't) get the respect of LED models from the 80's, there's still bargain's online if you like the older equipment, I grab as much as I can. I used to see the Panasonic 5000 Series all of the time, I quit buying after getting four or five of them, I'm sure they sold a ton and there's more out there. They do have an amazing tuner, it pulls in everything but overall just a normal box.
Bass is tough, we didn't have it back then, record grooves were limited and couldn't produce the pounding bass without making the needle skip. Most boomboxes didn't have the thumping bass that became more famous in the early 90's, I don't think any of my boomboxes would be able to thump, they can keep up with No Sleep Til Brooklyn, which should be the standard.