At first I thought of suggesting that it might be an attentuator that needs cleaning on the tweeter. My MS-802S have this adjustment and if the control hasn't been adjusted up and down in a while the control can be affected by dust or oxidation.
But being in the direct path of the HF source helps. I try to have my tweeters at ear level with the tweeters pointed directly towards my ears. I think that helps keep my perception of L/R balanced.
Distance from the speaker has a big effect. I have one speaker in the middle with a mono feed that I sometimes turn on. Moving that speaker forward or backward can have a dramatic effect upon my perception of loudness coming from the middle.