Okay to use RCA adapter on a fully balanced amp?

you are loosing what the amp is capable of if you are sure your amp is fully balanced.

So, with your amp you paid a lot extra because the manufacturer had to create two independent channels instead of just one, which ups the parts count and price considerably and you basically lose that benefit by sending it an unbalanced signal.  Use a fully-balanced preamp would be my suggestion.

You likely will experiience no difference. However, instead of an adapter, I would make or have made a quality cable with XLR on one end and RCA on the other.

@soix   You are assuming there is an inherent benefit to balanced cables.  Many, if not most, do not agree with that assumption.  Having multiple inputs=good.  Having ability to accomodate both xlr and rca=good.  Needing another RCA because your component doesn't have xlr out=common problem.

On a normal 1 meter cord, you won’t hear the difference, but on longer runs, XLR. 

Using an adapter as you stated in your original question technically will create additional noise on the line, I have no idea if you will detect the noise.