Well, I haven't actually talked to them about a sub yet (I will when I install the speakers later today). But I am certain that it would address any shortcomings in regard to lack of bass extension. Actually, I finished these speakers yesterday and listened to them most of the afternoon. While the bass was not quite as extended as with the vented HT1's, the sound was of very high quality and most enjoyable.
And the fact that they are sealed would really make integrating them with a sub a lot easier.
This customer currently has speakers that cost about $50. So they may decide that the improvement is so dramatic, they don't really want a sub. I will, however, recommend one and see what happens.
As for subs, I would expect to have some pictures of some sealed 15" subs with TC Sounds drivers in the next few weeks. We are currnetly working on a number of them for clients.
Then I would like to do a vented sub along the lines that we talked about a few months ago. If there were just a few more hours in a day...