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

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.

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.

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.

I listened to rotel under powered 200w and Mcintosh 600 mono blocks as that was the setup available at magnolia. I think i was comparing apple and oranges. I like the mids from mcintosh. Since it my first time buying amp and I live in Nj not many dealers have these amp wired to 803 D . how to i audition them any ideas and please advice

Your speakers are very revealing. You need to listen to whatever you are considering before you buy. Have you tried anything yet.

There are many brands that would drive your B&W's to perfection. McIntosh certainly makes some fine equipment (although I am more a fan of there amplifiers only). There are many names to consider... Ayre Acoustics is first on my list,Conrad Johnson, Modwright, Manley labs, Audio Research. These companies make great sounding equipment. There are a number of integrated amplifiers that you could consider from these companys as well. Last I checked Classe was making super expensive high power equipment ( top notch stuff ). hope this helps a little.