I've owned 3 yamaha romplers and all of them share the same basic sound.
It's not so much a flat or restricted frequency response as it is a lack of depth. I dont know if it's the dacs or whatever, but the sound has been with yamaha atleast since the CS6x.
The filters and FX have a plasticy quality to them as well... though the resonance has an interesting rock'n bite (esp on the BP & HP filters).
If you play to it's strengths things get cool though.
Try an RS7000 if you want the Yamaha-"flat" sound to the x-treme!
IMO: Roland Phantom has a lot of depth to them. Korgs Tritons seem to instantly get a vibe across once you drop it in a mix.