I would say that if your mixes are as described they would be a mastering engineers dream!!! I pray for those mixes that come in really loud because I don't end up having to compress/limit them very much at all in mastering because they are mostly tamed already.
I remember reading two different interviews with Bob Ludwig who probably has his name on more CD's than possibly any other person in the music business, most of the time as a mastering engineer. He said in both interviews that many of the mixes he recieves to master do not require much, if any, compression. They are loud enough. Usually it is a little eq tweek here or there to make it "perfect".
I have never seen a pre-mastered uncompressed mix that is "a little too hot". If you managed to get your mixes to "90% of the volume of a finished CD" (I assume you mean a mastered major label release) and the mastering engineer is complaining about that, you might consider taking your business elsewhere. I would say that the guy needs to check his ego at the door. Maybe he is afraid that you might not be able to appreciate the difference after mastering unless he is having to increase overall volume a whole bunch. Many mastering engineers seem to think that their only job is to make things louder.
There are a few on this BBS who offer mastering services. You might check them out for your future business as I believe all of them would die to get mixes from someone who has the mixes that close to finished. Count me as one of those.
Good luck.
Ed
