Good news. I finished assembling the new Mame pc and the ATI HD 4670 card performs excellently in it with soft15khz in XP going through a J-pac into my Taito Egret II. I first tried using Windows 7 x64 but couldn't get any of the drivers to work properly with soft15khz, so I gave up and went back to XP.
Currently, Catalyst 9.7 is installed. Can anyone confirm a newer Catalyst working without issues with soft15khz? I'm thinking of throwing on Street Fighter IV, and that game would probably function better with newer drivers.
Thank you very much, SailorSat, for all of the work you've put into soft15khz.