LDB,
If you have rung out your room before, and you have a consistent reference level to work with, then by all means, leave it the way it is. But... I can relate to what he's saying about 300Hz. As you probably already know, there's always the opposite effect, because you compensate for the dip at that frequency. My boy had these shitty monitors once, which didn't reproduce 7-12k very well, and dude's hi-hats could make your ears bleed.
When you have time, what I would do is: adjust your room by using a frequency analyzer, a good omnidirectional microphone, and a good EQ. But once you correct the problem to be flatter, now comes the hard part: Open all your old tracks that you've mixed in that room, and re-familiarize yourself with them. It's like you'll have to train yourself to get used to your room again. Although this sucks, it's like learning that your favorite artist that you've listened to for years does lip syncing. You'll get used to the new setup pretty quickly, and in the future at least you'll be listening to the truth, and not some rolled off sound at 300Hz to which your ears have become accustomed.
Also, nowadays everyone ultramaximizes and L2s the hell out of their tracks before mastering. It doesn't sound like this is your practice. It's good to hear that he said your mix was 70%. Usually I hear people complain about their mastering guy being too harsh, like, "Dude, you should really hire a mix engineer, I can't do anything with this." So this is a start at least. But if it's going to be mastered, back off a little on the stereo compression, and it usually helps. A great engineer will use no compression across the stereo bus. But even great engineers need it.
With that said, it's good that you know to keep everything at -6dB. Doesn't have to be -6, but if that works for you, then go with it. Although it takes up more space and processing, make sure you're at 24 bit. It gives you all sorts of headroom that 16 bit can't afford, and you get a much better signal to noise ratio if you have to back off to -8 or even -10 on some channels.
Anyway, hope this helps, good luck!