I love MIDI Solutions products and have had mostly good experiences with them. I did have a footswitch controller get wonky on me. I replaced it with one from the latest generation that can accept power from a 9V adaptor. So far it's working reliably.
The manufacturers, especially Yamaha, no longer consistently provide power over 5-pin DIN MIDI. So reliable power and ground isn't a given. The "MultiVoltage" devices are MIDI Solutions' way of working around this. Not apologizing for what could be a defective product, but food for thought.
Yeah, starting with the S900, the power coming from the MIDI out of my Yamhaa was insufficient to power my MIDI Solutions unit.
I never heard back from MIDI Solutions, but I've done another two gigs with the new MIDI cable, and I'm not having any problems. So I bought an extra MIDI Solutions F1 that I don't need. If anyone wants to buy one of the two. I think I'll keep the new one and sell the old one but first test it for a few more weeks.