Thanks to this thread (and antani), I was able to get 5.1 channel audio working in this fashion using my onboard sound card. In case anybody else is trying to do this method of room correction without buying the prodigy/ESI sound card, here is what I did:
1. Install Virtual Audio Cable (VAC) which allows you to route from any software player (at least the ones I tried) to another device similar to the directwire driver in the ESI/Prodigy sound card lines
2. Install ASIO4ALL (Free)
3. Install Console
4. In console, choose ASIO4ALL as the audio device
5. In the ASIO4ALL configuration, choose appropriate sample rate, disable all inputs and outputs other than VAC as the input and RealTek HD audio output (in my case)
6. Set ASIO4ALL buffer size to max (2048 samples) and zero the input and output latency compensation. Also make the kernel buffer size to max (4)
8. Open VAC control panel and setup one VAC cable with 6 channels (for 5.1 setup)
9. On the motherboard soundcard driver, enable 5.1 playback
10. Using Media Player Classic or other players capable of doing DTS/Dolby decoding or virtual sorround decoding, play the media file and choose VAC cable 1 (setup above) as the output
11. In console route the 6 channel VAC input device to a bass manager as in antani's posts above and connect the output of the bass management VST (Kelly Industries) to the 5.1 output device
12. Enjoy (I also configured the receiver to 6 channel direct mode to pass the 6 channel audio through to the amplifier)
I was able to make this work with any software player that can decode to 5.1 channels (Media Player Classic, VLC, TheaterTek DVD, etc). Latency is not too bad for audio playback.
Next step is to get the room equalized in drc...