barhrecords
Axe-Master
This is EXACTLY the question I've been asking myself but didn't want to post for fear of rolling eyes.
I did begin to think that maybe the guitars aren't so bright as you might think but everything surrounding it is actually less bright leaving your ears to perceive the guitars as being bright because they're perceiving a greater difference in presence . For example I've been listening to cymbals and have found that they are reasonably low in mixes, snares don't have as much attack as what you might dial in when listening to them on their own.
Here are two techniques I learned to analyze other people mixes.
If you have access to a DAW, import a stereo mix that you like.
First technique: Use a low pass filter with a steep roll off. Start with the roll off frequency high and slowly change the roll off frequency from high to low. Listen to when instruments "disappear" and at what frequencies. Basically rolling off the highs and seeing where instruments "live" in the mix.
Second technique: Start with the volume at a reasonable listening level like 83dB SPL, slowly lower the volume and note when instruments disappear in the mix. At the lowest volume note what instruments you can still hear. This gives you an idea of how the fletcher munson curve of your hearing vs. the mix are interacting.
Make your mixes behave the same as your favorite mixes by other engineers.