["jon_5912 I already have a Velodyne dd18, One thing that I know is that it's impossible to get flat bass in any room I've ever been in with a single sub and without a lot of bass traps and eq."]
I suggest using the TIP: on page 23 in the Velodyne DD User's Manual or running the Sweep Tones while walking (Crawl Test) your room and mapping the the loudest bass modes for a potential sub position.
If your DD-18 is positioned in your rooms loudest bass mode resulting from the TIP or Crawl Test with the mic at the listening position you should see a reasonably improved before and after result after running the Self EQ.
The initial goal is a relitivly even (not ruler flat) frequency response from the speakers 200Hz and the subs frequency adjustments beginning at 100Hz to where your speakers just begin to fall off. These are your crossover region adjustments which should be saved to your presets. Everything below that frequency are personal taste levels.
Unfortunately the manual LOW PASS, SUBSONIC, CONTOUR frequencies / slopes, PHASE and POLARITY parameter adjustments can be a tedious task using the sweep tones and confirmed with some music, also stored to memory.
Despite its twenty year old software these efforts will result in greater adjustability and detail compared to most any current onboard subwoofer processing today. Hardly impossible.
DD Plus does it all along with added discrete Q filter building automatically leaving some quick multiple drag and drop Manual matter of taste adjustments.