well, I did a project with the same problem and I fugged it up. So I'll tell you what I did so you can not do that (or shit; maybe it will work for you; who knows). I recorded to tape using dolby (C I think) and on mixdown; when I compared to other CDs (like fugazi) I noticed I was lacking all of these highs (next time I'll use GP9 and skip the NR). What I finally came up with was to blast the high shelf on the overheads. worked pretty damn good...it was a room recording and so boosting the overheads actually gave a little more "presence" to all of the instruments in the mix.
now, when It came down to mastering, and I began comparing my mix in the car to the radio....
I don't know why I thought of this but in my studio I took a commercial mix and raised the volume on the monitors to pretty fucking loud. I wanted to see how loud I could get it before my ears hurt. Then I did the same with my mix, and I could'nt get it nearly as loud....the low mids were freakin honking!! It was painful. And that's how I figured out that I had too much low mids. So I cut that during mastering. Actually I cut some low mids, some around maybe 40 hz, some in the highs (to reduce snare rattle and also to flatten out all of that boosting I did during mixdown), but I cut the low mids the most.
I got a much flatter mix. cutting the low mids actually brought out the highs somewhat. but what made the highs sound the nicest was cutting out the ones that were in the way. yeah, I cut highs, and it brought out the highs. I did this in the lows too.
Looking back I probably shouldn't have cut any of the lows. and maybe I could have done some of this in mixdown, not mastering.