I've got a Sharp GF9090 and an old Pioneer that both need the playback heads cleaning very often. In both cases the right channel becomes muffled loosing HF and overall level.. After cleaning with IPA it's ok again - This seemed normal to me but it's happening pretty quickly now and my tapes aren't that old and don't normally shed oxide badly.. Both tape heads don't feel totally smooth but look ok after cleaning - Could slightly worn heads increase shedding of oxide?
Are you sure the heads don't need realignment? Other than that, I would say if you have rough heads, that could be the problem. You're probably doing more damage to the tapes than the heads.
