N802 + Velodyne DD-18???

It is extremely unlikely that a sub would do anything to improve a dip at 50hz. A Rives Parc or a digital corecction system would be more likely to achieve the result you are looking for. Very few rooms are flat in the low frequencies. Just look at the in-room graphs in some of the S-Phile tests and see what I mean. So you could quit obsessing and listen around/through the aberations.

I just tried to correct some nulls in the bass (40/50hz)as heard in the listening position by using a sub and a parametric equalizer to supplement mains operating full range (down to 28hz). It didn't flatten (raise) those nulls appreciably, either electronically or by woofer placement without shaking the walls! I may have been sitting in a null but the walls must have been in a node. Compared with results that worked a little bit (less than a 3db change in the nulls) I preferred an amp with a better bass response. FWIW.

Thanks guys, but I also tried different amps, Behringer digital EQ and Copland room correlator. This equipment improves amplitude, but in same time you have problems with distortion, handling, not to mention converters who are responsible for all others things... My point is: I don’t think that anything will fill 10dB dip, but can contribute. I tried with lower class subs and I had better results if you compare with other mentioned experiments. Finally, N802 need some extra energy below 30Hz. At least in my room!

Srdan01, My room is about the same volume as yours and a DD-15 does fine for me just filling in below 30Hz. However be cautious, as current Vel 2.2 software only allows crossover at 40Hz and above. Unless you can get Velodyne to FPT you the older 2.1 software, you will have excessive overlap between the sub's LPF and a full-range main speaker. Using the sub's EQ to fill in null points much above the crossover point is usually a bad idea.

Any Vel DD can be improved with a few internal mods to the plate amp & wiring.

I actually planning to build new box for DD-18. Goal is to look like hi-fi stand (invisible) with down-firing base driver, like Rel.