Fletcher-Munson curves apply to each.
The most common mixing (and listening) level is around 85-90 db. If its material with a very specific audience, like rap or dance, get it right at 95-100 db or so. Classical and easy listening goes the other way.
If you got it right, the tone will be not be too thin nor too 'bassy' at either end of the range. Yea, it's one big compromise but you can minimize it. See how your mix translates to other systems and use it as a guide. Hit everything from boom boxes to your car to the TV. Compare your mix at different volumes to similar material too. You can also compare your mix to similar material listening from outside your room with the door shut. How does the bottom-end sound? Boomier in comparison? Weaker?
Another approach is to monitor and mix at a level that you can barely hear at. Now crank it and readjust to suit. Dial it in somewhere in between and hit the boom box, etc.
Regarding EQ: It helps if individual track EQ is done such that things fit together nicely. Same with individual track dynamics - it all works hand-in-hand. Then hit the entire mix with EQ to suit Fletcher-Munson requirements as you see fit.
One thing for sure - if your guitar track loses definition at low volumes you have too much gain and/or occupy more bandwidth that you should/could be. This doesn't mean you should only trim in the outer parameters of your chunk. Try some surgical carving inside and whittle it down to just what counts. That helps a bunch.
Compression has much to do with FM curves at the 2-mix stage as well. If the mix is glued at 2-mix only, the amount of compression required to do so usually doesn't play well across all the elements within it. FM curves don't like that much, causing an element (or more) in the mix to stand out too much at each end of the listening spectrum. I know you don't like compression, but when applied correctly (re: non-commercially) its a huge tool in dealing with this issue.
There are a zillion things that all add up in regards to the manner in which a mix stands up to the FM curves. Mastering is a big one. If the release is important, have it done by someone that knows what their doing and just concentrate on your mix. Leave the mastering guy plenty of overall dynamics to work with - just make sure the tracks work together well and things will be good.