Preamps for $300 to match with B&K st-140?

Sorry, I have to disagree with the passive and AI recommendations. The B&K ST140's rather low input impedance does not make it an ideal candidate for passive or high impedance out-put preamps. I tried a couple of passives with the B&K and didn't care for the combination. The B&K Pro 10 could be the ticket. The B&K ST amps can be a bit mushy in the bass, becarefull not to overdo it with preceeding "warmth".

Kool39, "the 10-100 times guideline" is some what arbitrary and IMHO generous. While 600 Ohms will probably work, it might require more attention to interconnect selection.

I'm not familiar with the diamondbacks. I would suggest avoididng interconnects that are very high in capacitance if you use a pre-amp with a high out-put impedance into the B&K ST-140. Of course the length of the interconnect will have to be factored in.

The truth of the matter is there are very limited choices in decent inexpensive pre-amps. The B&K pro MC 10 seems to satisfy all your requirements and would probably be the safest recommendation with regards to compatability. Please keep in mind that a new/different pre-amp may work differently with other interconnects. My expereince with B&K with regard to customer service always been very positive. B&K is a well established firm that should the need arise, will probably be there.

I just saw a Forte 2 listed for $250. IMHO worth looking into.