Plenty of decent subs out there, so to me, the most critical piece is what tools do you have at your disposal to perform the integration and know you are getting optimal results.
The first tool is knowledge, as understanding room acoustics is important to be able to select gear, place it and tune it. So read the
Harman Paper on sub placement, and how multiple subs can deliver even better room integration and performance.
Second is measurement tools and knowing how to interpret what they show you. These days, a simple smartphone app is a couple of bucks and has reasonable enough performance for setting up subs correctly. I highly recommend the Studio Six Digital app called 'Audio Tools' it's an all-in-one swiss army knife app with a bunch of different tests and metrics available.
More sophisticated user can get the
REW app for laptops and a <$100 USB mic like the
Daytom UMM-6
Third is some form of active crossover and time-delay to correctly blend the subs to the mains and set delay, so timing is in sync.
If using a HomeTheater preamp or receiver, they usually have those functions included, and if using the automatic room corrector (ARC, Audyssey, etc.) then pretty easy.
Otherwise, a MiniDSP 2x4 HD is a great tool and supported by apps like REW which can calculate and load crossover curves, and help you determine delay settings. Plenty of recipes on how to use this for subs out there.
If running more than two subs, then the MiniDSP 4x10 HD would be the tool.
With the above, you can take most decent subs and integrate them well to whatever main speakers and your results are quite likely to be better than just taking the 'best', expensive sub and placing where it 'looks' best and doing a basic connection.
I've seen setups with expensive JL-Audio subs that sound horrible, with super bloated, uneven bass. Not the subs that are the problem, it's the setup (or lack thereof).
In contrast, I ran an experiment where I took four of MartinLogans cheapest subs, Dynamo 300's, put them into the worst shaped room (acoustically) and it performs incredibly well. Here's the
thread detailing that setup.