Do you know what kind of power you will run to your sub stage? If you're limited on power (say only 300watt amp channel), get the Image Dynamics IDQv3. It's probably the most efficient from the bunch, but will handle higher wattage if you have it. It should work well in 1-1.2cu ft sealed box. SA-12 has much lower sensitivity than IDQ in sealed box and seems more suited for vented/ported box use. If you google on the issue, there have been endless debates on the topic of IDQv3 vs W6v2. I don't think they have ever reached any meaningful conclusion. Just flip a coin or buy the most affordable/suitable/available one of the two. Polk and RE seem like nice all around subs if you can't afford the ID or JL.
Actually, if I was buying for myself, assuming no cost is spared, I'd probably look into JBL WGTI or perhaps a used Morel Ultimo. These are considered the ultimate SQ subwoofers at this time. The Morel is ridiculously expensive and needs tons of power (1000watts?). It's reportedly better in SQ department than IDQv3, RE SE, and Boston G5. The JBL GTI can be bought for as low as $320 online and has very good sensitivity. 300-600watts would be fine for it (at 3ohms).
