best tube preamp under a 1000 for B&K 200 sonatas?

Its a hard price point even for something used. If you can find an old Sonic Frontiers SFL-1, that would be my first choice. Its one of the few low cost preamps that actually sound very good. Quicksilver is another good choice.

Be mindful of the output impedance of the pre.

Can you explain a quickly about the output impedance of the preamp? What should I stay away from? I have seen a few Carys on here in my price range.,also what about eastern electric min max?

You want the output impedance of a pre to be at least 10x less than the input umpedance of your amp, across the audible frequency band. To be safe, and since we often don't have measurements available across the spectrum, 20x is often recommended.

A nice used Quicksilver Line Stage would probably fill the bill nicely.