Suggestions for New/Used amp under $3500

I have a Butler TDB-5150 that replaced a Rotel RMB1095. Pre and Pro are Emotiva USP-1 and UMC-1. Speakers are Salk HT1TL's. The Butler brought my system to an entirely new level. Soundstage depth, air around instruments, instrument decay all improved to jaw dropping levels. Frankly I'm surprised that this amp does not get more press. Huge upgrade versus the Rotel.

Call Klaus at Odyssey. You may be presently surprised.

Based on online reviews, posts and personal suggestions, the following is my assessment of audio quality (not considering other factors like price, weight, size, heat, power consumption etc)

Emotiva < Odyssey < Butler < ci audio

Not sure where parasound fits in that ordering.

The Pass Labs X250.5 may be much better but does not appear to be within budget... even in the used market.

Parasound.

"Preamp outputs from a Denon 3311ci receiver."

Whoa. Stop right there. Is your priority 2-channel or home theater? If the former your first order of business is to address the preamp issue. The preamplifier section of even a decent midfi receiver will be a bottleneck for anything you're going to get out of your Unifield 2s. Better to get a good integrated amp or a separate stereo pre and amp and incorporate them into the system and let the Denon handle only the surround processing and amplifier sections for the center and surrounds for HT duty. This is easy to do and you can have the best of both worlds of a dedicated stereo system within a home theater setup. Let us know if you need more info on this, but the Denon as a pre thing is a non starter for good 2-channel sound. If stereo isn't a big deal and multichannel is your priority I think the Emotiva amps are a good option. Best of luck.