Mellow speakers for a digital piano

IMO, you're barking up the wrong tree using stereo speakers to amplify an instrument.  I would buy a pair of Roland keyboard amps.  I've been through this and two small amps gives much more spread and tonal weight than one larger amp.  Much more of the feeling of playing a real piano.  Also, they are much more subdued sounding than studio monitors.  I would get 2 of the ones with the 10" woofers.  $350 each.  

https://www.sweetwater.com/c525--Roland--Keyboard_Amplifiers

So ... you just set them on the floor behind the keyboard ?

Get a pair of 2 way powered P.A. speakers and a small pro audio sub. I use a pair of Mackie C200 10" wide dispersion speakers on stands in my studio rig (non powered, driving them with an old Alesis amp as it’s quiet…no fans), with a small Mackie mixer (PROFX8…el cheapo...it has an assignable graphic EQ as well as decent channel EQ and effects to play with if needed), and a now discontinued 92lb Mackie 500 watt sub (HR120 or something). My synth/piano sounds astonishing through this system…but you can get a little powered sub that will sound fine. Note that the newest "powered" version of these Mackies are lighter than my older, non powered ones (neodymium magnets), and, obviously, you then don’t need an amp…just any mixer.

“It takes $1800 worth of amplification to power some passive Dentons”

Definitely not and I was not suggesting that. Just thought I’d save the OP the trip down multiple rabbit holes trying to find the various components to mix and match and ultimately not meet his goals. Otherwise, there are probably infinite options out there depending on on budget and use environment.