[LAU] On the fly bass downmixing to mono (mpd or alsa)?

Hi, I started the thread "How is the bass mixed? Per-channel frequency analysis? Histogram?" a few weeks ago. One suggestion that came up a few times was to simply downmix the bass to mono before sending it out. How would I go about this? Currently, this system doesn't have any audio. What I intend to do is send audio out over USB to a DAC. Ideally, I'd like to do on-the-fly remixing of only "bass frequencies" to mono. Ideally the frequency should be configurable, but I think about 150Hz or below is a good start. If possible, I'd like to keep things simple, and have only ALSA and MPD running. As far as I can tell, I don't think I can do this at the MPD level. I saw an example where I can easily downmix all output to mono. I'm not opposed to that, but it would be nice to selectively downmix only the lower (subwoofer) frequencies. Thanks again! Matt