Just so everyone understands, assuming a quality tube amp FX Loop, eliminating ground loops during 4CM usage with the FM3 is a simple process, though it can require some extra inexpensive gear/cables.
It just comes down to whether the VP4's form factor and 4+2/4 simultaneous FX/Scene count is sufficient for your needs. If it is, then the VP4 is a no brainer, though if it isn't and you might want to use the amp models in conjunction with your tube gear, then the bigger Fractal products all do a great job at 4CM as long as ground loops and gain staging are properly handled, and that includes the FM3.
IMO, if it's possible, adding an IR Loader with HPF/LPF to the VP4 would be useful for those using loaded tube amps or tube preamps lacking IR support (silent stage, recording etc).
