One fine way to make a fun project pay off big is to make your own loudspeakers. In no other way that I can think of can the time spent be as rewarding. There is a huge return on investement when one choses to build his own speakers. Proper ones. Start here:
then, if you have the money:
There is always Dynaudio. I drive a pair with 1000 watts per channel...and we're talking genuine RMS wattages.... the kinda shit that will pop 20 amp mains breakers. I have a pair of Dynaudio Eurostat monitors. Bi-amped. Farking strong stuff. When I play BIG stuff, I point the brick work on my house. Again.