a superbass will sound distorted and lofi. it will max be about 15ow before it distorts

that wouldn't be much different. try tad's bass kit or weber's svt kit

I think having the right speaker is more important than having a high wattage amp. For example I have a Plexi 1987 lead which is 50 watts running on a 10 inch speaker and made horrible bass amp (distorts too much) but I put it on a 4x12 cab and the bass response became much better and I tried using it as a subwoofer to watch a movie and it worked great... (I didn't have a bass so I tried to simulate bass sounds by watching a movie with lots of explosions and stuff)

The bassman is really a bass amp but I really think it's the speaker that matters more, something that can handle low frequency and has a large enough cone area or movement to produce the frequency as well as good sensitivity. Try some 10 or 12 inch subwoofer speaker in a 4 speaker configuration...

Also with a guitar amp such as a plexi 50 lead I think a preamp in front of the input is needed because bass needs more signal strength... like an active pickup or just a  simple preamp that can enhance the low frequencies.

But seriously though, you're better off using a high wattage one channel PA amp (more than 300 watts) through a bass cabinet, loaded with subwoofer speakers or bass speakers. They're mostly solid state and should be way cheaper than most the tube amp kits anyone offers.