Mellow speakers for a digital piano

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

JBL used to make a very cool keyboard speaker that someone I worked with in the early 80s used. It had a shallow but tallish cabinet, a 15" woofer and one of those "bullet" tweeters.

Well, I have a mixer on the way.   I have my speakers on a swivel.  I have paper towel taped over my tweeters.

I am good to go.   LOL .....

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

This will teach you not to post questions regarding pro audio on a high end audio site :) :) I'm sure the resulting confusion was worse than before the question was asked :)

I was assuming this was for home use, and was going to suggest Wharfedale Diamond 8.1 or 8.2 which have no treble at all. Failing this the Q Audio lower range stuff has a dull sound...

Pro audio gear is typically not laid back sounding unless the tweeters are blown... EQ will change the sound. However if your keyboard is midi capable, you could use it with software that will let you choose the piano sound you like and apply processing to this sound, such as reverb, in the digital domain.