Instead of studio monitors I'd use a keyboard amp if available. They are different in purpose, so depending on the implementation they may work differently. Do you by chance know if the signal path of your studio monitors is analog or digital? In some rare cases, the digital-to-analog converters in a digital instrument like the FR-1x may interact less than optimally with the analog-to-digital converters of some digital sound processing unit.
I have the predecessor of the FR-1xb (and the x generation is supposed to work better) and the FR-1b works well for doing things like country waltzes (both accordion and some orchestral patches) but isn't really all that convincing with slow stuff with fine-grained dynamics. And the violins are awful either way, but the double bass is fun. I do end up using the FR-1b mainly for stuff that an acoustic will not do (silent practice, MIDI entry, low transport weight) but not picking it up when I have choice.