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>>>...The quality of the resulting MP3 tracks is pretty good. My first question is: is there anything I can do to make the quality even better ?...>>>

At the risk of flogging the obvious, I recommend playing back through the best speakers you can get. I use and recommend Logitech 3-piece computer speakers. I have them in every room in the house and i use them for gigs. Many pros of my acquiantance also use them for gigs.

They are no good for monitoring mixes though...they process the sound too much.

>>>...Are there any other clever tricks for mixing I should know about ?...>>>

Studio monitors do not have to be expensive. I use Radio Shack Miniumus-7 powered by a very good but old Kenwood stereo amp from a flea market. The Kenwood has lots of tone-shaping capability. The key is that it can be set to FLAT when I need it to be flat.

Flatfoot sez: Call me when 'Talent-in-a-Box' is ready to ship! -- [8{>

Got some tunes on You Tube:
http://www.youtube.com/user/flatfoot50
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My BiaB lesson site:
http://jdwolfe0.wixsite.com/learnbiab