If you're happy with the speakers,then concentrating on the amp is a good step forwards.
Asides from the Bryston amps mentioned,at your budget,you have a huge range of amps available,and more if you are willing to consider used/ex-dem.
I would generally avoid used valve amps,unless from traditionally reliable makes such as Audio Research,and similar.
Tubed amps to consider...Tube Technology(beautiful build and excellent sound for PP amps).
EAR make some beautiful valve integrated amps
Unison Research
You would probably want to avoid some of the lower powered SET valve amps,which also tend to be a little beyond your budget.
Solid state....this is where you'll find the biggest range,from used Krell/Brystons,through Parasound,plus all of the others such as Naim etc.
You would need to try at least a short-list with the speakers and system you already have,as matching an amp to your system is very important in terms of the end result.
Any decent dealer should be willing to do this for you.
Mixing pre and power amps from different manufacturers is an area where you would definitely need to try before buying...the results can be very variable,especially when the likes of input sensitivities,and impedance matching between pre and power amps are considered,even before considering how well their individual sounds may match.