What is under $5k speaker with best bass slam?

If your question also assumes used speakers, the Vienna Acoustic Mahlers, which can be found for as little as $4k/pair used.

I owned Mahlers for six years, have owned other large speakers and have heard a lot of large speakers - "slam" is the Mahler's forte. In each speaker, you have two 10" woofers, two ports, and two 7" mid/bass drivers. The 7" mid/bass drivers are the same 7" driver used in the WattPuppy and Maxx II's, they are normally used as bass drivers, and are, due to first-order crossovers, down only 6 db. at 35 Hz. (and that's just the mid/bass drivers). Largely as a result of the foregoing, the speakers are tipped up in the midbass and move a lot of air -- they'll pin you up against a wall.

Matti:

The Salons use 4th order crossovers, which ultimately allow them to go louder than the 1st-order slope Mahlers, assuming you have massive amplification to drive them (Salons are only 86 db. efficient, while Mahlers are +/- 90 db. efficient).

The Mahlers are a very different type of speaker, being intentionally colored to sound good with large-scale symphonic (i.e., rolled off a bit in the presence region, well-damped cone materials, fabric-dome tweeter, fat in the midbass, all designed to take the bite out of what digital does to string sections and to give body to wood-body instruments -- he didn't name it "Mahler" by accident). Having a peaky midbass, they are particularly exciting with rock/pop (... the WattPuppy formula).

The Salons are more linear and more extended, lower distortion, and often times, a lot less satisfying because of the nature of many recordings (not made for high-end equipment).

The real problem with Salons is that they require high-powered amps, and given that most high-powered amps sound like shit, well, ... .