I've found better results where I "master" individual tracks - be it with compression, limiting, eq, etc. And then mix down. Maybe this is obvious, I taught myself everything and none of my local ambient artist friends do much of any recording.
Which, brings me to something that has worried me and that is I've often thought about having my music mastered by a well-known ambient artist but I worry that a simple left+right mixdown couldn't possibly be mastered much better, if at all, than what I provide.
As you know, something that is mixed down well will need less (or nothing) in terms of final mastering. Creating an excellent final mix of a track is essential for a quality finish. This where the real work of shaping the music is done. But of course, that final mix of a track is based on your
ears and using your
gear, so has those biases. Having another pair of good ears listening closely
to that mix can be quite useful for noticing things you have a blind spot for. Could be a transition that is not quite right, a sound not sitting properly, or something going on too long, etc..
Mastering, on the other hand, is more about how the overall album hangs together. Making sure the flow of tracks is good, the tone of the tracks is consistent, the volumes from track to track are balanced. Its less about the individual mixes and more about the gestalt of the whole album. If your music is long-form ambient that already has a consistency among the tracks (as I'd say yours generally is) then the mastering part is probably less of a problem.
I'd say a primary reason for having a "well-known ambient artist" check out your album before release would be to have someone else you trust give it a very close and detailed technical
listen and give feedback on the overall mix quality and flow. I say "technical" because what you want is not someone commenting on whether they like the music, instead you want objective comments on the mix, and that is what the average listener is not going to give you.