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.

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.