Fixing leaky Delta faucet - what am I doing wrong?
Looks, and sounds like, the actual metal part of the valve seat is probably messed up from over tightening. It could possibly be fixed with a grinding stone that is fine grit and the same diameter, to reface the metal. But be careful to not take too much off or the seals won't seat properly. You might also look at the warranty that came with the faucet. It could have a lifetime warranty and be as easy as getting a replacement.