Yeah, the black ends on the green ones get gooey, too, over time. One of mine is like that, but I have another green one that - for reasons I can't explain - isn't.
The problem I've been having with original-issue Kabooms is (what else?) tape decks that quit. Currently, I have two cassettes stuck in B90 Kaboom decks, 'cause when the belts quit, the tape jams and the door won't open. This also affects the function circuit, so that, once a tape jams, you can't pick another function (besides "tape") without unplugging the unit and plugging it in again. It's a real pain! My later Kabooms (including the black & sliver B99) either don't have that problem or haven't reached that critical point yet.
Despite these issues, I'm still loyal to these darned things because the bass is so good. They really fill a room. Just got a DP-100 (the thick, stubby one) brand new in a box and love it to death, despite the fact that it's decorated with idiotic-looking flames and has a drum pad I'll never, ever use.
Plan to get an NB50 soon. Probably in white.