I have identified a strange and very frustrating bug in the PreenFM2's response over USB MIDI.
When my program (written in Java) sends a single NRPN 127/127 (data=1) over USB MIDI, most of the time my PreenFM2 box responds with *four* identical NRPN parameter dumps. I've been spying on the MIDI data results using MIDI Monitor and have verified that only a single NRPN request is being made. This is frustrating because the pfm2Editor doesn't seem to trigger this even though it's sending the same exact data.
Making things even weirder, when you connect the PreenFM2 over USB MIDI (on OS X, I'm running 10.9.5) it sets up a virtual connection TO ITSELF, so it's routing its own NRPN data back to itself, which is definitely not right.
I have also tried talking to the PreenFM over 5-pin DIN MIDI using a USB MIDI interface (a Tascam US-2x2). This works perfectly: I always get back one NRPN parameter dump per request of my program. The Tascam also does not set up a virtual connection to itself (nor do other devices, such as my Blofeld, BeatStep, or MicroSampler).
This tells me that with a fairly high probability there is a bug in the PreenFM2's USB MIDI driver or its behavior when over USB MIDI; because there is no difference between my MIDI stream and pfm2Editor: the only difference is that they have different USB MIDI labels. And further, the PreenFM2 behaves correctly over 5-Pin DIN.
This is a really frustrating bug.
