Rotel electronics will match well the B&W 600 series. The 604's can also benefit from stuff further up the audio chain.
I would probably go with an integrated with a $1000 budget.
On the used market for separates, one thing you could look for is a older Conrad Johnson PV-10AL tube preamp or PF-1L or PF-2L solid state preamp. They are all around $500. For an amp around $500 either McCormick or a Conrad Johnson MF2100 or Sonographe SA250
I would probably go with an integrated with a $1000 budget.
On the used market for separates, one thing you could look for is a older Conrad Johnson PV-10AL tube preamp or PF-1L or PF-2L solid state preamp. They are all around $500. For an amp around $500 either McCormick or a Conrad Johnson MF2100 or Sonographe SA250