Subwoofer meshes well with 2-channel?

30hz is low if that is at -3db
You will need a sub that is gonna reach 20hz or lower at -db to see a real significant improvement. That will mean no HT subs. Quality subs like bags end or rels bigger subs
The vandy mentioned is a good sub but not sure if it will get the job done with your setup

Apache1 may be correct about the Vandy 2wQ's in your system, but let me add two points that I should have stated in my initial post.
1. A pair of subs is much preferable to a single sub. Having two subs reduces or largely cancels out room nodes that may reduce the audible bass response.
2. The low frequency response you actually achieve in your listening room is going to depend to a significant degree on the acoustics of the room itself. Many people who add subs to their systems often find that "fine-tuning" the room (with furniture placement, sound treatment, etc.) is necessary.

What I can add about the 2wQ's is that I often play organ recordings and my 2wQ's reproduce frequencies into the low 20-Hz range. They also do a good job with low-frequency sound effects, although they lack the "slam" and dynamic headroom of some of the best subs (such as Velodyne, BagEnd, etc.).

Thank you for the suggestions.

I was thinking of a sub mainly from the standpoint of getting a more realistic "full" sound when reproducing full orchestral passages at higher volumes. These do go down pretty low, and I am not sure I need something that goes "lower" than 30hz, but something that moves more air may create more realism. For example, the speakers I could upgrade to (Tyler D1's) don't go lower, but they move more air (larger cabinets and 4 drivers vs. 2 on my LSS) which is why I thought that perhaps I could get the soundstage I am looking for out of the full-range D2 and still add a sub for realistic low-end.

I have tons of room tuning baffles around, so I bet I can limit any weird interactions. Just putting some panels in the corners really tightened up the bass.

I haven't looked into subs, but I assume they have built-in crossovers? Just plug the RCA/XLR's in from the Main 2 outs on my preamp?

I don't need a sub to replicate rock or hip-hop music. I just want my system to do a good job re-creating a double bass, for example.

Dwag you're looking for a sub that will go to 20hz clean. If you purchase a sub that only goes to 30hz @-3db you really don't gain a lot other then a little better control of the bass
Bass should not be heard but felt in the center of your chest for a two channel system

Sdcampell I have a great respect for vandy and have always been intrigued by their offerings for the past 20 years
I didn't mean to offend you in any way but I just think his speakers go to low and eliminate only but a few of the deepest reproducing subs out there

Vandys seem to work best with vandy speakersIMHO