Ah ha, I should have figured. I have an A-200 in my 6 and had exactly the same problem. It sounded fine on the ground, and at low power settings (low ambient noise) but at cruise it sounded awfully noisy in the headsets.
The A-200 does have adjustable mic gain and sidetone, but adjusting them is a pain. Remove the radio from the rack, and on top you'll see two holes labled mic gain and sidetone, with a piece of tape over them. Remove the tape on both, and there are little pots inside which are adjusted with a small phillips screwdriver. You make the adjustments and slide the unit back in and try it. If you've got someone to go with you it's not too bad to do this in-flight. Be careful with the pots as they only turn about 180 degrees.
I'd start by adjusting the sidetone down and leave the gain alone. As long as ATC reports that you sound OK, the mic gain is not the problem. I ended up adjusting sidetone down to about 1/2 of the factory setting before it worked right. If you turn it down too much you won't hear yourself in the headsets, but the world can hear you.
However, if ATC or whoever tells you the radio is noisy, try adjusting the mic gain down a bit until you get that right, then mess with the sidetone. Adjusting one effects the other. It's very much a trial and error process.
Jeff Point
RV-6
Milwaukee