Best amp for natural timbre of unamplified acoustic instruments & vocals w/ Revel F32?

For those speakers I would look towards a good quality newer Class D or Class D hybrid type amp.

Something along the line of the Rogue Pharoah or Peachtree's new Nova 300  (integrated amps) or Bel Canto ref600.

I have used older Bel Canto ref1000m amps with speakers having similar challenges for a number of years and remain satisified with those.  A pair of these used would certainly do the job for nominal cost.
  
These amps will do a good technical job of bringing out the best in your speakers.   If that still does not float your boat, then I would look to change speakers perhaps.

if your like me and are always buying used gear off craigslist i would have more than one amp , dac on hand for evaluations . Notice no preamp . 7 out of ten times the preamp is the root of what the op describes in my experience . Now a passive volume control has 100% worked with a dozen amps , dacs , speakers .What works with setup A  might not work when a new variable is introduced .

I have thought of Class D Bel Canto, Channel Island or other.  I might add also that I have switched around IC and speaker cables that I have and there is an improvement.

Marked improvement!   Tweeter break-in,  cable changes,  adjusting tweeter and bass settings on rear of speakers have made the Revels much more enjoyable!

Huge Revel fan. Kevin V is a genius designer backed with big bucks for R&D. Don't think he's capable of designing a bad loudspeaker.