Lack of bass in NHT 3.3.

I’m a 2.9 owner. The mid bass coupler and the subwoofer are room dependent. You have to have the speakers at least 7 feet apart with no furniture too close to the side firing subs.They also need to be close to the back wall and about 22" from side walls which I have mine and they sound good. If you have a TV between the speakers or furniture blocking the side firing subs the woofers are not loading properly and you might be getting cancellation of bass frequencies. Push the TV as close to the wall as possible so it don’t create a 2nd wall near the front of the speaker baffles or side subs. Get a CD that has lots of bass and play the music loud for days. Your woofers both the subs and bass coupler might not be broken in all of the way. The two pairs of connectors on the back try connecting the wires to the bottom set which is direct to the subs first in the circuit. Might be a phase problem. Make sure both the top and bottom connectors are tight, even the ones that have no wires attached to them. Try bi-wiring too. Not bi-amping but bi-wiring. Make sure the wiring is not out of phase. Check to make sure someone didn't wire the woofers out of phase to compensate for a center channel speaker not meant for these speakers. These speakers need to be tuned to your room and system like a fine instrument.

Also be sure to replace the deteriorated foam strip on the front baffle if it's rotted out. It plays an important part of the sound. 

you probably have a room null under 100hz somewhere. i have one at 45 and 75hz. every room is different, you will need to change your listening position or move the speakers. which is why ones that have strict placements are so troublesome.

There butt old but worth the effort. 
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I agree with the node suggestion above. I have NHT T6 towers (pretty much the 3.3's replacement) and the bass is not powerful unless you are in certain spots in the room. Being that they are tower speakers, they don't offer the placement flexibility of a monitor and separate sub - you're kind of stuck with what you get. I did add several standalone subs in the room to help balance things out. The woofers are only going to get down into the mid 20's before they start rolling off, so subs will be useful anyway to get the very bottom frequencies.