What is the best Amp/pre Amp for B&W 803 diamonds

Audio Connection is in NJ. He sells your speakers and the Rotel. I've been to audio stores all over the country, and he's the best one by far. There's some big stores where I live here in NYC but I would say, don't waste your time.

Rotel is probably a good choice for you. B&W and Rotel are part of the same company so its a pretty safe match. Since you're new, you need to do some listening. Over time, you are going to learn more about audio and your taste in equipment will change. For those reasons, I would recommend you wait before you spend a lot of money on electronics.

Classe CA2300 is a good match, which I have used for few years with 803D/Diamond speakers. Smooth, detailed, neutral. Bass extension is very good, but not as good as e.g. Devialet.
Classe preamp+amp will exceed your budget unless you consider use a good cheap DAC with volume control - but test compatibility.
Another option is Devialet all in unit - superb sound for modern compact package.
Also I previously used Marantz successfully with 803D - consider the PM11S3 or PM14S1. Marantz reference test speaker is 800 Diamond, so their components are well matched and voiced to drive the B&W range.
Finally just want to highlight that the 803Diamond is a superb speaker that rewards every upgrade or tweak - well balanced, easy to listen, and reasonably transparent. Sound quality also improves with running in and will improve up to 6 months.

Contrary to the conventional wisdom, I have heard 802s and the 803Ds sound very very good w a good tube amp. Particularly a push pull EL-34/6CA7 amp, making about 50wpc. You would need one w a very very good power supply but I would not rule tubes out.