There are subs out there that will do great off 300W even for the pair. The JL W1v2 series come to mind. Sound good. Very efficient. Work well sealed.
The problem that you will run into is Hoffman's Iron Law. There are 3 aspects of a sub, efficiency, LFE and small box size. You get to have two at the cost of the 3rd. Want good efficiency and a good bottom end, the box is going to be really big. Want good efficiency in a smaller box, the bottom end is going to suffer. Now it doesn't have to be a total trade, you can compromise and get good results.
Back when I was first getting into car audio, the standard tradeoff was get efficiency and mix and match the other two. This was because power was so expensive and big amps were really hard to come by and even harder to support electrically. Some of the tricks used were to go with an efficient setup in a smallish box and let the cabin gain give you some free bass on the bottom end. That was the most common way to go in fact. The other common setup was a nice free-air setup as you were talking about. Really big box, but totally non-intrusive. The result was a really efficient sub system with an almost limitless bottom end and no real lost space.
Nowadays, power is cheap. Efficiency is almost sworn off in all but the high end audiophile type drivers. The typical consumer doesn't care about efficiency anyway because he only cares about what the thinks he knows. Ask someone not really up on car audio about their subs and usually they know brand and how many watts. That's it. They automatically think that more is better in the watts department. Most of them only want to get as loud as possible and it breeds the standard crap that you see day in and out on here regarding "you NEED to run the RMS rated power to a sub for it to perform," "you NEED at least 1kW on the sub stage for it to sound good," etc... Fact is that neither of those resemble the truth. Regardless of the rated power of the sub it will sound just as good with a fraction of the power and get more than loud enough for most sane people.
I might have missed it before, but what have you done to couple the trunk to the cabin? Which way is the sub currently facing in the trunk? Have you measured the voltage coming off the HU? I had a similar situation to you with a trio of 10W6s. Amp was working fine, in fact the clipping light was flickering, but no output to speak of. Scratched my head for close to an hour, rechecked all my settings on the amp and HU 3 or 4 times before I realized that I had in fact wired all the coils on the subs out of phase. It's easy to screw up and do on accident. Also possible that one coil is mislabeled, or did we already cover that...all these threads run together after awhile.
I just saw your last post: don't put much of any stock in sub efficiency ratings. They are theoretical calculated values at 1kHz. Subs with lower ratings usually get lower and do a better job on the sub-bass.
