And as far as your audio shop story goes... Remember, you're dealing with a salesman. Their minds don't operate like normal humans... It's all, "dollars and cents in my pocket," to them. As far as getting the thing loud, it's gonna be in the enclosure. As far as going loud, and staying mainstream Kicker and Alpine are the two brands that come to mind. Either an L7 or a Type-R are rugged and can get some decent numbers in the correct enclosure, though they don't offer much in the quality department.
A 15" sub I've been checking into is the Mach5 MAW-15. Fits your bill quite nicely as far as price, power, and mix of SPL & SQ. Another option I would place in the exact same category is the Dayton Reference HO 15". Neither of these subs are going to win you competitions, but if you build the correct box, the output is going to be incredible. A bit more of an SPL sub to consider would be the AscendantAudio Arsenal lineup or the mid-line RE's.
And IMO, I would steer clear of the HiFonics stuff, and stick to solid amps from brands like: USAmps, Alpine, Kicker, Sundown, ZED, TRU, (upper model) Phoenix Golds, (upper model) Orions, Rockford Fosgate, etc. And you're MUCH better off upgrading the wiring rather than placing a band-aid with a capacitor.