EQ is the electronic software that tailors the in-room response of all the speakers so it is flat (or as flat as can be) across the frequency range, rather than being a peaky, boomy mess due to room-modes etc
It's usually called Audyssey or YPOA (in the Yahama receivers) for example, but can also be a computer-based system IIRC.
If you did go with a sub, you could cross the mains over around 100Hz or so, taking some pressure off them and letting the sub take the strain of reproducing the lower frequencies.
For $5-6k, you could build dual Stereo Integrity 24s (http://stereointegri...4-24-subwoofer/) in a nicely finished box or two (depending on whether you built 'dual opposed' or singles) and an amplifier with suitably crazy power to run them, perhaps one of the SpeakerPower rack amps (that don't have noisy fans IIRC). You'd also be able to afford an EQ solution to lift the bottom end as required and tame any peaks caused by your room, and probably even have enough to upgrade your power line to a 230-250v line (assuming you are in the USA) to ensure you had no restrictions due to power supply..
Of course, if you don't want to DIY, you could look at the Deep Sea Sound subwoofers:
which are run by a member on this forum, @dgage:
They use the drivers above but come fully constructed and nicely finished, and have plate amps built-in.
You should also look at some of the stuff that BossoBass does, who is also a member on this forum:
I think he is somewhat at the higher end of anyone's budget, but he majors on flat response down to 3Hz, so that should give you all you ever want for movie playback.