If you have tried swapping and eliminating all hardware (checked voltage output on standard tone out of each amp etc) then clearly it can not be the equipment. It is you or the room. If you know for sure others hear it (or you have tested your hearing with same db output from each speaker and you dont hear the difference) than it is the room. If it is the room, try: (i) measuring dbs with same db tone CDs, right and left channel using an audio shack db meter and if that does not make the issue clear (ii) download the room EQ wizard (buy a mike) and measure not only immediate dbs but also map out various milisecond scenarios. That will allow you to see if reverbs are causing sustained dbs beyond the immediate timing (often the problem.) If that is the problem, get your money back from your room designers and start experimenting with room treatment (forget equalizers for reverb problems, much less helpful if at all.) If that is not the problem, then move houses!
By the way I assume your issue is not only vocals but pretty much all sounds. Otherwise, it is really mysterious.
By the way I assume your issue is not only vocals but pretty much all sounds. Otherwise, it is really mysterious.