I am recording a symphony orchestra in our next concert. I currently only have a stereo microphone ( audio technica 4047) and portable field recorder (Tascam DR100). It's been suggested the item that would help boost the quality the most would be a good pre-amp. Can I get some guidance on what is best for live orchestras - so warm and precise is what I'm looking for. 4 channels would be best because I might rent two extra mics. Thanks.