I think whatever Midi accordion you have at this day and age may not matter soon. For example' the BK7m (which I think needs to be upgraded), is the sound solution to make any model accordion sound good. The iPad using Midi LE (midi to Bluetooth) is the wireless solution to an arranger keyboard or BK7m. I am doing this now with Songbook+ app ( pdf music reader), MidiJet (wireless Midi), Yamaha MD-BT01 ( Midi to Bluetooth transmit/receive) and Line 6 G10 (wireless audio).
Connected presently with my Korg arranger. Works perfect and gives freedom to stroll while controlling the arranger.
I know evertime a new model comes out, sales go up on the older in hope of investing in a new model. Save your bucks!
I just received the FR4x and yes a better sounding accordion then my FR7. I bought it because it is smaller as my main reason. Although, on my fr7, I Midi transmitted the right hand of my accordion to my Korg PA3x accordion sounds (which I think sound real good).