Barry,
> Putting both slot absorbers in the room, we still have our 120 Hz room mode, with maybe a dB or two drop in level. <
I have to admit to no experience with slat absorbers. So all I can offer is this:
Passive bass traps made from 4 inch thick rigid fiberglass are known to be excellent at 120 Hz when mounted straddling corners. Since this type of bass trap is so easy to build (or cheap enough to buy), maybe just cut your losses and switch to that type of trap?
Also, adding only two bass traps to any room is not enough to make a big improvement at any frequency. A room has hundreds or even thousands of square feet of reflecting surface, and each trap is considerably smaller. To make a meaningful improvement in the response and ringing you need to cover a meaningful percentage of those reflecting surfaces.
--Ethan