Hmm...lots of midrange drivers (esp 5") have a rising frequency response between 300 and 1k or so, which will thin out a piano,sometimes with a "cuppiness" coloration on human voice, too. For that reason I've preferred 6.5" midranges as a rule for inexpensive speakers. I've designed both 5" and 6.5" two-ways, and have a Steinway B in my room as a reference, and could NEVER get the 5" to sound as neutral as the 6.5. (Yet a GREAT 5" CAN be made to sound neutral, of course, as was accomplished Verity Audio in their Parsifal Encores, for example). Also note that room reflections around 800-1k are VERY position sensitive. Try moving your listening position just a few inches to see if the mids even out. That may tell you to play around with sidewall-damping schemes like sofas/pillows (what I use), etc. I do NOT buy the CDP suggestion as the source. There are other issues ("digititis", etc.) with CDPs, but a rising frequency response in the midrange isn't one of them!
Good luck.
Good luck.