Here's what I've found so far:
In the ASIO device listing using BASS_ASIO_GetDeviceInfo, I get two possible devices:
0: Device:Creative ASIO Driver:C:\WINDOWS\SYSTEM32\CTASIO.DLL
1: Device:SB Audigy 2 ZS ASIO 24/96 [DCC0] Driver:C:\WINDOWS\SYSTEM32\CTASIO.DLL
If I select the former, I apparently have up 22 inputs and 20 outputs available, and with the latter 6 and 8. This link decribes how they are mapped in the former case: http://forums.creative.com/showthread.php?t=279862
It's not entirely clear to me whether I can achieve what I want to with this system. From the link above (and assuming that all channel numbers are +1 from how they are enumerated in BASS):
ē ASIO inputs 3 and 4 (Mix Pre EQ) This records a simple downmix of all audio coming out of the sound card, without EAX effects, regardless of what speaker mode you are in. (This isnít a physical input);
!! My NOTE: (this functions as "What U Hear")
ē ASIO inputs 5 and 6 are "Mix (Post EQ Front L/R)", and this, too, isnít a physical input. Using compatible software, this allows the signal to be fed via the onboard Creative DSP Spatializer and main EQ, making it possible to record a "virtual" 6.1 mix for playback over a two-channel headphone or loudspeaker system. What happens is that the DSP Spatializer processes the individual surround components in a way that recreates the illusion of 6.1 surround-sound from a two-speaker system. Very neat! Whatís more, you can hear the effect when replayed over any two-speaker system or headphones, and so, if you use this effect on one of your own tracks, anyone can play it back using regular stereo playback equipment. Note, however, that, if youíre in 6.1 speaker mode and record this output, no signal will be output from the rear, centre or subwoofer channels. What you hear will be the virtual surround-sound coming from a single stereo output. If you want to hear these elements separately, choose "Mix (Pre EQ)";
I'm guessing that one of these two options might be what I want, but it's not clear to me whether I can also be generating audio on the surround channels at the same time.
A problem my intial experiments have shown up is that I don't get 'digital silence' when no stream is playing with either of these two options. Maybe the line in and mic inputs are also unavoidably 'in the mix' ..? Can I turn them off? I have also seen this when using lesser sound cards which, apparently only have an analogue mixer, but I thought that the Audigy 2 was all digital.