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

How about an Adcom 565 preamp. These sell for between $200.00 and $300.00 used.Known for it's neutrality..it will probably do the trick.

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 bought a Dared SL-2000A tube preamp last week. I purchased it for a second system that I don't have together yet, so I put it in my main system. I have to tell you if you can get away with just 2 inputs and no phono, this pre will be hard to beat in that price range. It can be had for a little over $300 brand new(I payed $330 total). It comes up weekly at auction here and on ebay. It even has a little keychain remote.

I need a preamp with good bass reproduction. Do the preamps you guys have suggested do that?

A preamp can only give you whats there to begin with.Also the sensitivity of your amplifier will be a factor.I haven't used the Dared on a B&K. But from trying it with a modified Dynaco ST 120 60 wpc paired with Solilquy 6.3s and Paradigm 20v3s paired to an Adcom 585..rattling the windows wasn't a problem! The preamp cannot make up for deficiences in the source or the amplifier...unless your using tone controls, which I don't use. Tone controls can only do so much, it will always be at the mercy of the amplifier.