I lost most of the videos I took of the Sunday night concert, so I don't have a lot of examples, but I do think a lot of the problem with the guitar sound on your clip, Kendall, had to do with the way the acoustic guitars were miked.   The guitar I used (thanks so much, Nancy!) was acoustic and Charlie had to keep adjusting the mic position while I was singing and playing, and it still came across weak and tinny (not typical of Martins) in the video of my song. But when I videod unamplified instruments in the cabin or louder instruments in the concert, the instruments came over better.   Everyone's voice was well amplified in the concert, and the voices tended to come over louder than a lot of instruments, including your guitar.




Check out this clip of Mick, voice and guitar unapmplied, in the Peace Cabin.




All the videos I did were done with the same recording equipment, which was just the built-in camera and mic on my MacBook. Differences in sound quality result from things like how close the mic is to the sound source, whether and how the sound source was amplified, and the acoustics of the room itself.   
There are tools that will let you tweak sound after you've recorded it, but I don't have anything like that except for a playback volume control and a "normalizing" function (that basically evens out the volume highs and lows). I can't adjust things like bass and treble or add reverb, etc., at least not with iMovie.

(One of these days I'll experiment with my Garage Band to see what its gizmos can do.)