Best bass response in under $1500 used towers

I third NHT 3.3s (or 2.9s under $1000 used).

Paradigm Studio 100s have 7inch drivers.
NHT 2.9s have 10inch.
3.3s have 12inch. It's not even close in terms of bass response between 3.3s and Studio 100s.

Stratus Goldi towers (with dual 8.5inch?) have alot of output/SPL down to ~40hz....comparable to the 2.9s (which go a little lower).

Here are two detailed reviews on the 3.3s:
http://www.stereophile.com/loudspeakerreviews/1293nht/index.html

Soliloquy 6.3
They have a bass response of 25Hz. Not quite as low as the NHT (22Hz), but something to consider depending upon your choice of amplifiers, and preference in sound.
Good luck in your search!

If you stretch your budget just a tad, you may be able to pick up a pair of Thiel CS3.6's, which are not only very full but also very accurate in the low registers.

add a grand and take the time to find a pair of used dunlavy cantatas, better yet some athenas but the cheapest i've seen is in the 2500 range. they'll be the last speaker you ever need