Barnabas:

I have one sub now which is based on Infinite Baffle design vented behind the screen. It is connects directly to a Behringer 1400 watt amp in bridged mono mode, which does have Balanced inputs on the amp. So with a 990 I would have Balance from the 990 --> FBD --> AMP.

See: http://www.behringer.com/EP1500/index.cfm?lang=ENG These run about $300 a unit and since I use rack mounting they are ideal.

I am thinking of adding an LFM1 if I get a 990 to the second LFE output and using the FBD to handle both.

The FDB is an awesome unit. It allows you to flatten your sub response for your room and also "notch out" frequencies that are problem areas, like windows, heat ducts, or areas that are boomy because of room modes. They sell for $99 online, which is way too cheap for a unit like this.

We don't all have a anechoic chamber in our houses..

You will need to setup a PC with a mic to measure your frequency response. Since I build my own speakers, I already had that stuff.

A really good reference site for FBD and Infinite Baffle subs can be found at: http://f20.parsimony.net/forum36475/index.htm