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

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

any comparison advice Krell to Curl?

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

What about the Parasound Halo A21+?

What about the Bryston 4B3?

You will like the A21, best buy in my book. 

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