Variable gain amplification for speakers

Okay, I'm new to this forum, so try and give me a break. Here's what I want to do....I want a device that I can place at any distance from a speaker (within the confines of my living room) that will send a test signal (either pink noise or a variable tone generator) to the speaker and adjust the gain accordingly to match a preset level of, oh, let's say 70 dBa. Basically, something like an SPL meter with a feedback to the amplifier. The desired end result would be something that could potentially adjust the levels of all the channels in a 5.1 surround system just by placing it in different locations in the room. I was thinking probably an omni-directional electret mic and then some kind of variable gain amp that feeds the tone generator or pink noise generator. Any ideas????

Are you sure your 5.1 doesn't already have a built-in tone generator for speaker balance - all I have ever heard of have this feature on the remote control.

Yes, that's correct. All Dolby Digital and Pro-Logic receivers have a built in tone generator. But I'm pretty sure they all also require that the user adjust the gain for each channel manually. I'm actually talking about a stand-alone device independent of the receiver. Forget the statement about the 5.1 system. I just want a circuit for a single speaker that will send a test signal to the speaker, collect the speaker output through a microphone, and adjust the gain to match a preset sound level.