Some of you are may be familiar with dbx. It's similar to Dolby in that it reduces hiss, nearly eliminates it actually, but moreso it's a compressing/expanding (companding) process that greatly expands dynamic range. There are some drawbacks such as "breathing" during quiet passages and finicky about recording levels. I used to record CDs back in the day with dbx on my old Aiwa deck, but unlike Dolby its unlistenable on a non-dbx system.
I did some searching a couple years ago and actually found one boombox with dbx, but cannot remember how I found it or what model it was. I know that it was black, a later model probably made in or after 1986 by one of the big names, and was either a 3-piece or looked like one.
It's exceedingly rare among portables. I only know of three models. Two are Walkmans - the Panasonic RQ-J20X and the Teac PC-7RX. The Marantz PMD-430 also has it.