Sub Suggestions

Once again, the ML Depth. Very forgiving of placement when using two, and terrific in all other ways. Has separate LFE input.

I would seriously consider the Onix Rocket UFW-10. Small beautifal sub that is very good for home theater and music. Very good reviews at Goodsound and Audioholics. Website is AV123.com. I just purchased one.

Doesn't Aerial make a sub? They'd likely match your Aerial speakers.
-Bill

Why do you need two subs?

A single Aerial sw 12 sub could be all you need . Two is better of course as each could be used for seperating bass for each channel but the unit has a switch for summing the input. It also has a switch for movie and music and you can optimize the sub for both. Other than that , the SW 12 is a fantastic sub and would mate nicely with your speakers. Beautiful , near indestructible and harmonically rich ; music reproduction for this unit has few peers. For theatre , it is very good to excellent.

velodyne spl 1200r's sound pretty amazing!

I think the Rockets have a parametric EQ on board also, yes? Not sure on that, but think so. That would help as well

I want something smaller than the Aerial SW12 and I don't want to spend that much given that it would only be used for HT and sit there 98% of the time doing nothing. I like 2 channel music most, everyone else wants surround sound. But know that this mode will be rarely used for the most part.

The Rockets do have parametric EQ on board, which is very helpful when setting them up. If music is most important to you, I think you will love it. Very compact also.

Rel Acoustics makes some excellent subs. I don't know what the Aerial SW 12 costs, but the Rel Storm 3 can often be found right here on Audiogon for around a thousand, or so. I own a Rel Storm 3 and love it. Quadophile has written an EXCELLENT review (right here on Audiogon) of the Rel Acoustics Strata Mk III. Here's the link to it:
Rel Strata Mk III: Review by Quadophile

The Rockets are a nice value but they aren't in the same league for sound quality as your Ariels.

A better match sonically would be something like used RELs, or used Nola Thunderbolts. These go plenty low for HT but are better than most for music as well.