When talking about adding another Roland keyboard to a yamaha arranger, you could also opt for Fa06 or Fa08 which are both cheeper then bk9 but have more to offer soundwise.. More supernatural acoustic sounds, super natural synth sounds, an awesome arpeggiator, 2 virtuall expansion slots... 6 knobs for direct controll over your sounds.. A nice sequencer, and a delicious little sampler

However, its either 61 or 88 keys, and does not have arranger functions.. But then, you allready own a yamaha arranger..