LR, I'm a big fan of live music so for the last 35 years I have worked to reproduce the live leading edge drum attack, guitar decay, etc. that live music presents but is very difficult to reproduce in a living space.
I've had many quality subs, parametric EQ's, receiver EQ's, etc. over the years and nothing sounded anywhere near as realistic as what I have now. In the end I found simpler was better.
Two years ago I purchased a pair of HSU ULS-15 subs, their flagship subs. I have to say these subs are outstanding and having a pair has virtually eliminated troublesome room induced frequency peaks and dips. Bass is very uniform and it did help the low end on music.
When I purchased the subs I was aware of the Mid Bass Module from the HSU website. HSU however really promotes the MBM as an HT item and not so much for music which I think is a mistake on their part. I was able to find only a few obscure user reviews of the MBM online related to music but those few were very positive.
Given the generous HSU return policy I ordered a pair of Mid Bass Modules and got a quantity discount to boot. The difference between MBM's only and subs only is profound. Of course the MBM's and subs together sound wonderful but 85% of the wonderful comes from the MBM reproducing the 50 to 150 Hz range. So if I had to choose between the MBM or sub it would be no contest in a music system.
Dr. Hsu explains on his website that true subs are designed to go low and loud (typically well below 50 hz.) and for this you need a large heavy driver. He also explains that what I call low end whack & attack resides in the 50 to 150 hz. range and for that you need a smaller, much lighter and faster driver. A true sub just can't do it "ALL" right at 20 hz and 150 hz.
After finding this link it was interesting to be able to visualize how music material resides in the 50 to 150 hz range. Hearing the Mid Bass Module makes this even more apparent provided your upstream audio gear is up to the task.
http://www.independentrecording.net/irn/resources/freqchart/main_display.htmSo LR I can say that the HSU USL-15 subs are fine if not exceptional subs and they can not "pull off" what the Mid Bass Module can do. Seems like their designer Dr. Hsu believed the same when he created the Mid Bass Module. And I have never enjoyed my music system more.
I will admit I was a skeptic but as they say "Hearing Is Believing".