As a developer I have access to the RTM version of Windows 8.1 and decided to install it on my primary system. Largely it's been a great update with very few problems. However, my Line 6 drivers are having issues with my Pod Studio GX.
At first, L6 Monkey would have trouble seeing the GX. When trying to do anything with the device (selecting as an input in Reason, changing the volume in Windows), the system would temporarily hang. I went through several rounds of reinstalling/clearing/installing the Line 6 drivers and programs. I'm to the point now where L6 Monkey always sees the GX but no input is working.
Hopefully Line 6 releases an update for 8.1 in time for the official rollout in a few weeks. I submitted a support ticket to bring the issue to their attention. For now I guess I'll use my amp or laptop.