I'm currently using the 3604 without any problems with my i7 2600K @4.6Ghz with 1.33v and the PLL works just perfect. I didn't use the 2001 BIOS, so I cannot say if that BIOS causes any problem.
In the worst case you can rollback without problems, but 3604 it's very stable and didn't have any issue at all with the rest of the hardware, don't worry
ASUS Products: Maximus X Formula - RT-AC87U/AC88U