The ARIA imparted more energy in the upper bass region, which seemed to smoothen (i.e.: make more flat) this area compared to the SOtM DAC-direct connection, and moreover spatiality and/or the sense dimensionality was more pronounced/convincing, which was especially apparent during live applause sequences that seemed to emanate from a plane just behind the speakers with striking realism - quite impressive. The uptick in upper bass energy also made voices come through with a bit more solidity.The most common problem with operating a passive control is the loss of bass impact; in this passage you are confirming it.
What I don't get is why you stopped with the Aria. There are a lot of preamps out there, and they really vary! IMO/IME it is a statement about how bad many of them are that a passive system can keep up with them. Getting a sub adjusted correctly is always a tricky bit- and I know David Belles well enough to know that his electronics are really flat- so the bass you are referring to above was not being enhance by his preamp. It some other interaction, and setting up the sub to compensate is creating a synergy.
IME again, synergies are usually a situation where one weakness is played against another, resulting in something that *seems* better than either on their own. An alternative is that if you worked with stuff that stood on their strengths rather than their weaknesses, you create something that is even better. I would try a variety of preamps and see if you can do better that the Aria, and be willing to readjust the sub in that light (since you were willing to do that to compensate for the usual loss of bass otherwise...). You might also look into tube preamps; IMO they offer more detail than solid state.