I found this Polish music shop's Feb 26th comparison of the RD-800 and the CP4 useful - you might appreciate it too. Would be interesting if you hear things the way I do...
2:35 Playing starts. RD-800 V-piano-derived patch.
This sounds like a REALLY close mic'd piano. Attack is a bit metallic to me.
2:51-2:55 I like the slightly evolving sustain here.
2:56 Even at medium volumes, still a somewhat metallic attack to my ears. But my personal preference is for a bit more woodiness.
3:41 CP4 patch 1 starts.
Seems like "mic placement" is a little "further away" than the Roland.
3:58-4:01 Maybe I'm reading too many Dewster posts
but I swear I can hear the static looping in this sustain. Doesn't sound quite as organic as the Roland.
4:02 Attack seems a bit more realistic than the Roland to me. especially in the upper registers.
4:33-4:36 Same sustain "plainness" as earlier.
4:57: RD-800 NX concert grand
4:57 A less artificial attack than v-piano derived patch. More to my taste than the first one.
5:09 As he pounds the keys the attack gets a little more metallic -- but the the dynamics are really big! (very present bottom end)
5:48 Same powerful low end as prior patch. However, something about the balance between low and high end seems off to me. Maybe just the huge stereo width?
5:55 CP4 patch 2
6:06 Rich and pleasant low end, but doesn't have the dynamics of the Roland to my ears.
6:43 I like the low/high end balance much better here than on the Roland.
6:57 RD-800 Studio
I can see why this gets a lot of praise on this forum. Woodier than the prior Roland patches, but with the same nice "moving" sustain.
7:43-7:46 Again, high note decay seems overpowered by the thunderous low end... I like the Yammy's balance a bit better (8:42). Perhaps he didn't hit the high keys hard enough, and could be addressed also by changing the touch curve?
This was a lot of fun for me, and helpful as I am shoppng for a DP and plan to spend a lot of time soon in local music stores testing them for myself. They both sound quite nice, and as usual it will come down to personal preference.
And BTW, as a long time lurker, this is one of the best forums of any kind on the Web - helpful, insightful, respectful.