I am fascinated by your technique of mixing with an old boombox and I want to try it...
Is it possible to know the exact name of yours to use as a starting point? There are so many boomboxes and I don't know which one to get...and yes, it's just to use it as a monitoring device...
I have a constant Google search for the model number of my little radio. I've never managed to find one.
It's a cheap radio cassette from 1996 and I'm guessing mine is the last one in regular use anywhere in the world.
I only bought it because back then there wasn't a huge choice of powered multimedia speakers available like there is now.
Just get a cheap set of PC speakers - they'll do exactly the same job. The model is unimportant as long as they are reasonably neutral and of adequate quality. Not too good, not too shit. Nothing with a subwoofer.
The trick, as with all monitors, is spending time learning them and getting to know what they're telling you.