Bill's right. The demands/design parameters are very different. If you're a jazzer who wants a mellow, clean tone, a bass amp may do the job. Otherwise, not.
Flightcase 150 is very neutral/clean sounding. 5 band eq - subtle but effective. Mine does a great job. When I need more volume, I add power amp and cab/cabs - it has a pre out. Very happy with it.
In common with FinnDave and Hiram, I'm crazy. Bought a new Mex P Bass Special last year which sounds excellent and plays very well. Had to sort a few minor issues - dress the fret ends, adjust action, etc - but I expect to do that sort of thing with any instrument.
If you have a gigging amp, Markbass may be overkill for home use (although they will certainly do the job and sound nice at low volumes). Then again, it's nice to have something decent if you can afford it (my practice/home amp a PJ Flightcase). Try a few before deciding?
I sold a head a few months back. Buyer asked what sort of guarantee. I said none, am a private citizen, etc. Demo'd and got him to declare himself happy before taking. End of story.
I'm a Gotoh fan. 201 mainly, but they do the 203, which is a more solid version of the traditional BBoT, which works well. Have one on my fretless. Don't like the Badass - seems to rob the instrument of warmth.
As others suggest, you could stick with current band for the social/fun side and do something more satisfying too. Have you actually tried saying to the current band "love you dearly, chaps, but cannot carry on like this". Might wake them up to the need to improve. Or are they a lost cause?
I reckon Sunburst Jazz's suggestion is the best one. Why not make a filthy low budget vid' with the old GoPro along the lines he suggests and email it to your rivals? Should make 'em think a bit...
I built a cheapo P - Squier body, no name neck, s/h Kent Armstrong pickup. Cost about £250. Does the job and nice to play. Pleasant change from the vintage Jazz.
You can make foam bungs to fit a lot of round ports with sponge paint roller refills - available from DIY stores. They have a small hole through the middle for the roller shaft, so don't completely block the port.