I was looking at doing this on a PC, and found that an application called "Console" that allowed an easy way to use VST plug-ins in a fairly lightweight host application. You could, of course, use a more sophistocated DAW application as well. In my case on a PC, I used the RME's "Loopback" feature to send the Media Player's output back to an RME Input - then Console takes its input from there. I split the signal into 3 stereo chains, and applied filters and delays (VST Plug-ins of your choice) as needed for L/M/H outputs (each stereo chain goes to a discrete output pair). On a Mac, Core Audio might allow this w/o using RME's "Loopback" feature, but I dunno. You'd still need some kind of a host to insert the DSP AFAIK.
There might be a more streamlined way to do this, but I have not seen any pre-baked solutions to allow Loudspeaker Managment and Processing on a computer with a multi-out soundcard (this could be a killer niche software market IMNSHO - basically like a Driverack or even a Lake processor as a "plug-in" or standalone host that would work with any multi-output interface ).
MADIface-XT+ARC / 3x HDSP MADI / ADI648
2x SSL Alphalink MADI AX
2x Multiface / 2x Digiface /2x ADI8