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

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. 

The Revels are not that bad, I heard these F32 with a Pass integrated and the sound was not thin, hard, or sterile.  I also have experience with your preamp, and here in my room, IT was the culprit for this type of overly analytical, hard, glaring sound.  I tried the finest vintage tubes and expensive cabling, no luck.  Finally moved to a Joule Electra preamp and enjoyed it much more.