Just one more followup - even with the (removable) casters, I decided I might want to stack the slant cabinet on a standard cabinet at some point, and would still need 4 rubber feet to do that properly, so I contacted the local Line 6 "service center" nearest me, and asked about ordering some replacement rubber feet. They don't carry them, and suggested that I call Line 6 and see if I could order them direct. I called Line 6 Support, explained that I'd been all over the parts listings on the website *AND* contacted my local service center to boot, and ultimately, if anyone could help me, it would be Line 6. The agent was very helpful, and gave me the following information to order replacement rubber feet:
description: Rubber Foot for Spider 412
Source: Full Compass (www.fullcompass.com)
1-800-476-9886 (ask for parts department)
Full Compass has to order them from a vendor, and waiting time is ~4 weeks, but since I'm doing the casters anyway, I can wait for them.
Line 6 doesn't sell direct to customers, so this is apparently the only way to get replacement parts that aren't on the website or available at the service centers.