Mellow speakers for a digital piano

Seriously, try out a reverb. Its highly likely that is part of the sound you are after.

If this is just for home use, I would start with a tube amp of about 30 watts, like a Dynaco ST-70 or the like, and find a set of speakers that are easy to drive so that 30 watts means something.

My JBL JRX215 speakers that I use for my keyboard rig are 8 ohms and 97 db; they are easily driven by a 30 watt tube amp. On stage I use 500 watts per channel, but there is no need for that at home. You can find them for $300 each on ebay. They are not the last word in resolution, but they sound surprisingly good with a decent signal. Not much output below 40Hz.

You might also consider a set of JBL L-100s...

You might want to process the sound a bit. I would not use a mixer, but I would consider a digital reverb like this one: http://www.alesis.com/products/legacy/nanoverb2

A lot depends on what you are planning.

If you are playing only at home, stereo speakers are fine. If you plan to play in a band or on stage, pro audio speakers and amp(s) are required as you will be playing considerably louder (and likely less concerned about the overall sound, so long as its undistorted, has the bandwidth and is loud enough).

You might consider a tube amp if you plan to use the piano only at home. That could easily take the sharpness out of the high end without loosing any highs (this being that often sharpness is an artifact of distortion of a kind that is common in solid state amps). Tube amps are usually best used with speakers that are fairly easy to drive- 90db+ sensitivity and 8 ohms or more, particularly in the bass region.