So who makes the best modern small enclosure sub?

Credence still makes the original "Solobaric" subs. They made them for Stillwater. So I would give them a look.

Image dynamics works well in a small sealed enclosure

Dayton HO series works well but you might consider porting that thing as they realy shine in small ported. They sound very fine doing it.

I would suggest avoiding the L7 in small sealed. I have seen very little success going that route.

Amplification is cheap now days. A mono Class D amp can be had for under 200 bucks in the power class you are looking at. JBL makes a quality product and their factory refurbs are a fantastic deal. I bought my sons 2 1500 watt sub amps as factory refurbs and he has been abusing them for the last 2 years. If you have a preference for brand or aesthetics look for that. I like the plain black amplifier. If you like more flash, I would have a hard time recommending something.

I have had fantastic luck with Sundown audio. They are a boutique brand that has catered to the SPL crowd but they just happen to make a dang good product. I tend to work on several systems a year as a hobby so if you have a specific brand set of questions, I can try to answer.

10 years and still in production: The Infinity Basslink. It's an intergrated powered sub of about 0.84 cubic feet and 200 watts rms power. It has a "servobaric" design with one active and one drone 10" speakers. The active driver has a feedback winding for the amp which results in extremely low distortion. Typical subwoofer driver distortion is 3-5%, which kinda renders the difference between an amp with 0.001% distortion and 0.1% a moot point. But with the active servo design reducing distortion, the Basslink has less than 0.3% total distortion at full volume, which is 1/10th of the typical amp/driver combination.

I'm a believer, I have bought 3 BassLinks so far.

Along the lines of what cubdenno said: Image Dynamic does well in small sealed as does Polk's MM line (which I've got in my 350Z). I've got a Kicker L5 sealed in my truck and its never really impressed me, they tend to do better ported and with lots of power.