Well said, Chris B (Born again Scouser) You absolutely need a pair of independent ears to listen and suggest. Most musicians can't hear beyond their own performance on a track, even though they might think they are being detached and that definitely colours whether they think the track is good, bad or indifferent. And you're right in that it is very hard to concentrate on playing well if you are also trying to make sure levels, volume and etc are all as they should be.

There is nothing worse than listening to a piece of music further on down the line and hearing forever the bit that makes you wince..:-)