Thank you for that suggestion. I had the ASIO4ALL on there because my M-Track Plus installation instruction suggested it to reduce latency. I went ahead and uninstalled the ASIO4ALL and uninstalled, then re-installed the driver for my M-Track (there wasn't a driver for Windows 8.1, so I used the Windows 8 version). Now I'm running into another problem though. When I start up Pro Tools, it tells me that "the ASIO audio device could not be initialized. Please choose another device." When I go to the next screen, it says "playback engine" on the top, with M-Track Quad ASIO Driver below it. I click 'ok', but then it tells me the exact same thing, "the ASIO audio device could not be initialized. Please choose another device." I'm stuck.
Any other suggestions?