How good are you at fixing electronic gear?
That's what you are going to need to master in order to fix your subwoofer/amplifier.
Did you read that little rectangle with the two triangles on the back of the unit? The days of yanking the tubes out and trucking to the local Radio Shack to test 'em are long gone. Unless you have some idea of what's going on there and the necessary tools to troubleshoot this stuff the odds of you fixing it are slim to none.
Just looking for bad parts rarely provides the answer. And, if a part is obviouisly bad, many times when it dies either another bad part caused it or it takes other parts with it. Many guys have replaced transistors only to haver them blow again instantly upon powerup because another part caused them to blow in the first place.
The guys who do fix this kinda stuff for a living have knowledge, training and experience and deserve to be compensated for that. If it were that easy they wouldn't be needed but alas, they are. I'm pretty good with tools but you won't see me asking a dentist how could I do a root canal with a B & D variable speed drill and no training.
As far as the other speakers, I suppose you could use then with a receiver but understand that they will probably bo very lacking in bass and attempting to force them to reproduce serious bass may cause them damage. The bass box(it ain't a subwoofer) and amp combo probably did a "Bose" and diverted the bass from them.