I’m one of the users quoted here, and I can give a little more info on my experience. I have a bunch of Mesa heads, and I’ve used the FM3 with those and a few other brands as well (EVH, Synergy, Victory, and PRS). The FM3 is a stellar unit and it can be used competently in 4CM and will make most people happy. I just noticed the sound of the amps was not quite as good as when I ran my amps with other FX processors, notably the HX Effects in a similar 4CM configuration. I also did not like that I had to run an XLR to 1/4 cable out of output 1 to get 4CM to work correctly.
Moving from the FM3 to an FM9 for the same function all of my concerns dropped. The FM9 sounded better, and I could use standard quarter inch cables for the connections. The FM9 is immaculate as a 4CM option IMO. For the FM3, I can’t fully speak to the converters and I have no scientific tests to show results from, just my ears and experience using a lot of amps and effects processors over the years, I was not fully satisfied running the FM3 as a 4CM effects-only option.
I have a VP4 on order and can’t wait to try it. I am mainly looking to use it in tandem with my amp heads either in the loop or in 4CM mode while my FM9 remains connected to my PC as a utility device for writing and recording. With the recent FM9 updates, especially DynaCabs, it’s really become the most incredible all-in-one writing and recording platform, replacing several of my amps and pedals.
Curious, how were you monitoring, guitar cabs, monitors, studio monitors?
Did you match the gain and the monitoring SPL between the FM3 and FM9 while testing?
I have a AFX2 and a FM3, and both sound fine 4CM with any number of amps, i.e. Fenders, Mesas, Peaveys, plus many tube preamps including; AS_JCM800, AS_Twin, Kittyhawk Quattro, ADA MP-1, Marshall JMP-1, plus custom MTS tube preamps; Orange OR120, HiWatt DR-102, Matchless Chieftain, Dumble ODS, Fender Deluxe, Vox AC30TB, Marshall Plexi, Friedman HB. With the tube preamps I use the Amp Block for tube power amp sims. Note that of course all amp FX Loops are not created equal, so that's a variable.
I've monitored them in pro environments through Urei, Weslake, Yamaha, Digidesign, and Genelec monitors, as well as a variety of guitar cabs and headphones/IEM's, so I'm really confused why people say the FM3 sucks at 4CM, especially vs the AFX2 which I also have years of experience with which doesn't sound any better IME (I've no experience with FM9/AFX3)?
What you're saying is that there is freq colorization or dynamic changes in the dry path which I find hard to believe assuming the FM3's audio setup is correct (for example my Les Pauls with Duncan JB's in the bridge require the 18db pad to avoid rounding the waveform, so one needs to test this via a DAW!), but as soon as you add FX that pretty much becomes a non-issue anyway. As to the balanced output needed to break a ground loop, that is certainly possible and can be easily corrected by any iso-transformer without requiring a XLR connector as well.
It's interesting that so many folks are ragging on the FM3 for 4CM use, though IME it works fine, so I'm thinking it's related to gain staging and ground loops etc, or perhaps some badly designed amp FX Loops.