B&W 804S + $2,000 used integrated

In addition to the Musical Fidelity A5 or A5.5, I might suggest some:

Tube Amps

Pathos Classic One MkII
PrimaLuna DiaLogue 2
Cayin Audio A BBT

Hybrid

Vincent SV-236 MK II ($2000 new)

For solid state I suggest

Creek Destiny
NAD Masters Series M3
Bel Canto S300

Something from Classe or Blue Circle if you can find them in your price range.

Hybrid

LSA or DK Integrated amps.

I've owned a lot of B&W's, altho not the 804's. They tend to like a lot of SS power; & I'd think either of your choices would be very good. Plinius could also be well worth checking out.

What's your budget? And new or used?

Well, I've narrowed it down to either a Musical Fidelity a5.5 or a Vincent SV-236 MKII.

Can those "in-the-know" please explain to me the basic differences in sound I should expect between the two?

Also, would these be compatible with the B&W 683's as well?

Both are excellent and well reviewed, the MF being a powerhouse and the Vincent having a tube preamp. One of the Marantz PM integrated units might be a good choice as well.