Memantine was unhelpful, it may have even spiked me. Now have to wait the long half life period to get it out of my system. Was worth a shot...I have Memantine but the long half life freaked me out if things went wrong. I tried very little so can't say if it did much.
I tried DXM... similar idea, much shorter half life. it did reduce tones considerably to silent levels... but left some underlying "background" wavering sounds. Ultimately not super helpful. It got really loud at one point but ceased upon stopping within a day.
If you tap the back of your head to reduce the tinnitus with earplugs in you'll hear what I mean. Sounds like bells.
If you don't hear it, lucky you I guess. I always have some of that around.
Anyways, on DXM that bell effect was extreme. Given Memantine is also an NMDA sodium channel blocker I'd expect similar symptoms.