Benobs

I just added an XLS2000 to my HT system to power a passive sub-woofer I had laying around. My main speakers are powered by a Rotel Integrated amplifier.

My concern right now is the mono signal required by the XLS in bridge mode. Right now I have a work around by using the dedicated output from a sound card, but my next upgrade on the list is a quality DAC (stereo only). No need to specify that I use unbalanced outputs.

Do you guys have an idea that could save me some headache?

Here are my current alternatives :
1) An external "mixer" that mix left and right into a mono out. (Any suggestions appreciated)
2) Go stereo and plug each voice-coil (dual 4ohm) into 1 channel. (I dont think its a good idea...)
3) Parallel the voice-coil (total is 2ohm) and use 1 channel only (not sure I will get enough power)
4) Make an home-made passive stereo to mono adapter with resistances to protect the DAC. (Ouput level may be to low)
5) Sound-card.... (Currently, no sound-card seems to fill my needs in terms of quality and flexibility...)

*What happen when you run the XLS in Stereo Mode but a speaker is plug in bridge?

Thanks all!