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

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.

Both of these amps are excellent and I think you would be happy with either. The MF is going to have a little more control over the woofers and perhaps a little better detail retrieval, while the Vincent is going to have a little more tube bloom and warmth. Either one is going to be very good with the B&Ws, depends on your preference. I have only heard the MF with B&Ws in auditions, and it sounds great. I think the Vincent would be easier to listen to over the long run if you are at all adverse to solid state treble etch, while the MF is going to be stronger on rock and classical recordings that demand extreme dynamic swings. Hope that helps.

As for the 683's I think that they would be well matched to either of these amps, with similar caveats applying as given above. The 683's are not as defined or precise as the 804s (or as interesting looking in my book), but may provide some great enjoyment with these amps and associated sources. If you are looking for heirloom speakers that could serve you through this amp upgrade to several more, then the 804s, are your choice.