$8oo dollar budget

speakers are B&W 602.5 s3, Room size is 20x25.

Agree with Tvad, tubes will have trouble driving those speakers becuase of the impedance dip and swings and maintaining balance from highs to lows. That B&K is a pretty good amp for driving those speakers. I love tubes and only use them in my system, but not all speakers will mate well, yours are not likely to be a match made in heaven with most tubes amps, there are exceptions (Music Reference RM9SE) but they will be expensive. At your budget, I would stay away from tubes, unless you change your speakers first.

I would try to find a pair of b&k m200 sonata monoblocks as they sound tube like but I agree with Tvad and Pubul57
tubes at 800.00 budget would be hard to achieve.

I agree with Tvad and Pubul57 that your current speakers preclude most if not all tube amps in your price range.

One possibility is to go to a Class A solid state amp. It won't sound the same as tubes, but may give you some of what you're looking for. I had good success with the Monarchy SM-70 Pro, using two in a monoblock configuration. They'll handle the 3 ohm dip just fine. They can sometimes be found within your budget for a decent pair. Make sure they're of similar vintage so that the input sensitivities match.

David