The "boom" you are referring too is not positively correlated with the quality of the sub; in fact it's inversely correlated with sub quality. The more ACCURATE a sub is the less "boom" there will be, unless the program material calls for it. If you crave inaccurate audio, you may miss the "boom," but I cannot see this being useful in any type of music other than techno, hip-hop, and rap. People who listen to these types of music generally over-emphasize the pseudo deep bass frequencies from around 30-50 Hz (which is where the perceived boom is. This isn't true deep bass, however. My subwoofer produces near triple digit SPLs at 16 Hz (plus it's very quick and it's fairly linear even without my parametric EQ - but I still use it to compensate for room modes). Now that's a deep sub - MUCH more rewarding than some pseudo deep cheapo sub, but mine took quite a bit of work in selecting components, hiring a cabinet builder to build the box to driver specs, etc. Not everyone needs this level of quality, but I prefer it.
Also someone mentioned above that two 6.5" drivers do not have the surface area of a 10" driver... This is not exactly true. Two 6.5" drivers would have a surface area roughly equal to that of a single 10" driver (the same as a theoretical 9.75" driver), but typically the excursion on 6.5" drivers is less than that of 10" subwoofer drivers. Because displacement (excursion * surface area) is what matters in deep bass, the 2x 6.5 will be slightly lacking, but this all depends on the quality of the drivers. It is technically possible for 2x6.5 drivers to be SUPERIOR to a single 10" or even a 12" driver if the excursion is enough on the smaller drivers.
"In terms of deep bass, there is no replacement for displacement."
