1) You need to be mixing with a subwoofer in order to check what's going on down bellow 100Hz. It is impossible to know what's going on down that low if you can't hear it. If you don't have a subwoofer, you can temporarily turn up the low frequencies on a stereo speaker pair and check what's going on down there. Be careful that you don't blow up your speakers, or wake the neighbors.
2) Before I turned on my sub speaker, I was impressed with how well I could hear what the bass, vocals, and guitars were doing. I really liked the tone of the guitars in relation to the vocals. It is possible that you can't hear what's going on down bellow 100Hz, and if this is the case, you have a really good mix in the freqeucy range above 100Hz.
3) Now for the producer's comment. It is critical that the peak frequency of each instrument compliment the song as a whole, and what I really liked about YOUR mix was how well the vocals, bass & guitar fit together - specifically how their frequencies fit together. When you get these details correct, you bring something special to the mix.