Vandersteen 3A sigs, whats wrong here.

Second Pmotz' generous reply. Even a seasoned user can get phase reversed accidentally :o( and much more important, positioning and room characteristics have a huge effect on bass.

One approach is to use monophonic source material with a lot of bass content. Turn the balance control all the way to one side. Place that speaker vis-à-vis the rear wall for best bass output. Then center the balance and place the other speaker for imaging. Yes mono for imaging: the sound should be centered.

The process takes a while; apologies if I'm telling you what you already know.

reposition the speakers closer to the walls.

Are there really speaker cables out there that are so bad that they could possibly cause a speaker -- one that is well known to have excellent bass -- to sound like it has "not much bass at all"? That's a sad state of affairs, if true.

I vote for a phase issue. With 10 connectors, it would be easy to accidentally swap a pair.

certainly the out of phase issue should be looked at.

Daverz,

I just mentioned the wire because I do think 8TC is a bit lean and if a few other factors like setup and one other lean component (if applicable) could sway the sound to be thin and lightweight.

I have found system matching to be critical and balance can sometimes be difficult.

my two cents...

Alot of great answers above, if one of these do not answer your question, its possible a speaker problem. To put this all together, Start easy, check your speaker wires at the amp and the speakers, + to +, - to -. Next, temporarily move your speakers closer to the walls/corners, see if this brings out the bottom, next, you might want to slightly back your mid and treble controls down a hair on the back of the speakers.
My guess is as mentioned you are dealing with room nodes. You might want to do something like measure the room and use something like the "Golden Triangle" formula by Cardas for imaging. Placing speakers by its measurements do help get rid of room nodes. I suspect that you have gotten used to a bit of bass boost by your other speakers being closer to the walls. You can use the Golden Triangle, then if not enough bass, start moving your speakers, by a couple inches at a time toward the corners until bass reinforcement is where you want it, then adjust your tow in accordingly. I hope all of this helps. Tim