Small Sub, NHT Sub 8 or Velodyne MiniVee

The question pertains to my bedroom system. I have Kef LS50 speakers and a Hegel 80 connected to a Sonos connect. Bass without a subwoofer was good, actually better than I expected, but I wanted to try a sub. The room measures 13 FT x 15 FT.

I have since added a NHT Sub 8 (rated to 38khz). It is placed in a lower shelving unit with open sides and back. Space is tight, but it works (less than 12" square). I think the sub has been well integrated, adding some fullness to the lower end without being pronounced. The volume is set at less than 50% otherwise the sub sounds too pronounced/out of balance.

Would adding a second NHT Sub 8 further enhance the sound, broadening the sound field? The other option would to change out the NHT for a Velodyne MiniVee. The MiniVee plays lower (28khz), and has a more powerful amp? Any thoughts between the two options? The 3rd choice would be to leave it alone, knowing it sounds good the way it is (albeit, alway wondering if it can be even better).
The LS50's are good enough for which to get a great sub, not just an okay one. The Sub 8 is fine for a pair of Super Zero's (which I also have), but the LS50's rate something like a Rythmik F12 ($899).
Hi. Difficult to answer your question. I can imagine a number of ways how a bedroom setup is used, and that would probably have a significant impact in what the better approaches might be.

Do you listen from a stationary position, or do you move around? If you move around, read the papers by Dr Earl Geddes about using multiple subs. His main goal is to achieve a flat bass response at different points in the room by using multiple subs. This is very important if you listen while you move around much, or if you have more people listen with you. If one is stationary, then optimizing for one given spot can be done too. Geddes argues for the use of 3 or 4 subs. I've tried with two, worked very well, but lately moved away from the approach because when I listen critically I do so from a single point, stationary.

Expanding the soundfield is usually first optimized by proper speaker placement, and listener placement. I struggle to see how that can be done in a bedroom. Maybe in your case it is, but generally speaking I would suggest to let go.

I'm not familiar with the subs you mention. A sub that goes lower is great, but it must go low in a nice way (low distortion, tight). Larger drivers provide more impact as they move more air. Tough to get around basic physics, despite what marketineers might try to make you think. I have two 12" subs and wished I had gone for the 15" even though my bass is very, very good.

I used to have a Rel Storm III and replaced with two Rythmiks and couldn't be happier. Despite being much cheaper than the Rel, the Rythmik drivers are larger: 2x12" vs. 1x10", so not fully useful for your comparison. I like two subs, though.

BTW, please note you are missusing khz, which means kiloHertz, or 1000 Hertz. So 38 khz means 38,000 Hz, which is not what you meant. 38 Hz is what you meant.
Thank you for the responses and the correction on the terminology. In terms of the soundstage, the Kefs sound great. It is the fact that lower output from the sub doesnt feel as expansive as the rest of the music. If I raised the volume of the sub, though, it then sounds out of balance and you can pinpoint the location of the sub. It is a small sub (8") and that is why I thought a second sub may help the lower end sound more full.

I have space limitations, and can only accommodate a very small sub(s). I purchased the NHT to see if I would like the addition of the sub in the system and it is a convincing yes. The sound benefit is subtle, but noticeable, albeit it doesn't match the soundstage of the Kefs. Other than try the Velodyne MiniVee (plays lower & more powerful). Another option would be two Rel T-Zero subs. The spec is similar to the NHT (38) and the company is known for musical subs. The cost of 2 Rels would approximate the cost of the higher spec Velodyne. Thanks again.
I bought a used REL Q150e a few years ago for 200 bucks...goes down easily to 25hz, sounds amazing, and one does the job in my medium-large room with aplomb...aplomb...
I went with the Velodyne MiniVee. The MiniVee is really impressive. It easily outperforms the NHT Sub8. The sound has more volume, while still be tight and clear. In my room it seems to be perfect with LS50. It adds richness to the lower end and it blends seamlessly with the monitors. At this point, I think my bedroom system is more enjoyable and musical than my main system.