Aha yes! Playing the mix at home. Familiar problem! You may find that your home speakers are knocking a lot of the 'top' off the voice, and may also be boosting certain rogue frequencies - which could make the mix sound totally different to how it did in the studio.
Has your engineer got some cheap monitors for checking the mix? Speakers can make more difference to the sound of a recording than the effects used in the studio.
It's important that they use both top-range (for accuracy) and rubbish speakers (because many will be listening on cheap speakers - specially radios) to check the sound.
Then when he gives you a mix to take away, listen to it on as many different CD players as you can find: Car, walkman, beatbox, hi-fi - you'll be amazed by how some intruments appear or disappear. This is why people use compression and aural exciters etc. - so the music will sound kinda ok regardess of the system it's played through.