Used monitor under$600 with good bass low levels

Triangle Titus (very small with clean if not deep bass) or larger Triangle Comete or
Ohm L (bookshelf) with Ohm sub-bass activator tweak.

Whoops, sorry Triangle Titus cannot be biamped and Ohm L is 3 not 2 way.

Both are good, smaller box designs for well under $600 used though.

Ejlif, Mapman, thanks for the recommendations, I am still curious as to what the ohms are like. I am hoping someone in nyc invites me over.
Have you(plural) had a look at the bolzanos.

PEdrillo,

If you think there is even a chance you might keep them, you should do the in-house audition of the Ohms...there is no risk involved if they do not work out.

If your listening space is tight you might be satisfied with a pair of small Micro Walsh Talls for ~$1000 or so, particularly with the existing sub. I think the Atmasphere amp could work really well with these smaller Ohms. I can also vouch that the Denon 103R even on my comparably modest table sounds outstanding on my Ohms!

The Triangle Titus ES has twin input connector sets and can be biwired or biamped. (I have a pair.) So can the Comète ES I imagine.