New/Used Amp recommendations for about $3k-$4k?

What about the Parasound Halo A21+?

What about the Bryston 4B3?

I like John Curl’s design criteria and circuit topography over Bryston.
I would prefer 2 x used JC1’s over a new A21+ for similar money. Then if you can get a cheaper used A21+ for even less ??? but there’s no high Class-A bias switch like on the JC1’s and that’s a BIG + for me for serious listening.
They also have switchable input sensitivity from 50mV to 250mv perfect for unity gain active preamps or passives pre’s, like the Schiit Freya and Freya+ that have all basses covered passive, tube, or solid state.

Cheers George

any comparison advice Krell to Curl?

Earlier D’Agostino designed Krells yes, later not so great.
Cheers George

The stereo's will be fine with that loading of around 1.5ohm, but the bridged mono's will not.
As bridged amps only one advantage, "more watts" but all other parameters that make an amp great go backwards, especially stability and low impedance drive capability.
Cheers George

I’m very curious about those GAN amps mentioned above.

Great if things stay above 3ohms, but your 100f’s don’t, they won’t bring the best out of that bass area like the JC1’s could. 

Cheers George

todd1010 OP
I’m looking some new Paradigm Founder Series 100f speakers and would like to get rid of my Emotiva UPA-1 mono blocks for something else.

The 100F’s drop to 3ohms around 100hz, but!! are also -45 degrees phase angle there too, which means they could be as 1.5ohm EPDR load as seen by the amp.
https://www.soundstagenetwork.com/images/stories/loudspeakermeasurements/paradigm_founder_100f/imped...

https://www.soundstagenetwork.com/images/stories/loudspeakermeasurements/paradigm_founder_100f/elect...

If you want the very best out of them in that bass area, suggest an amp that won’t flinch at those kind of bass loadings.
My suggestion for your budget would be used John Curl’s Parasound Halo JC1 monoblocks. They can be switch to high bias sweet Class-A sound for serious listening. And low bias for other listening, rock or background.
https://www.stereophile.com/solidpoweramps/774/index.html

Cheers George