my sl 1210 is louder on the right channel

Are you using tubes? If so anywhere in the chaini and they are not well matched this will cause the issue as well.. Only way to find this is to swap tubes from channel to channel, I found this with a Phono amp utilizing 12AX7's once, we thought and did everything with a turntable and speaker placement and figured out it was one tube being as little as 3% off will show problems in balance, for some reason especially with analog sources even more so over digital.

Of course try all above suggestions as well as it could be any one of them, but tubes thrown in the mix is also a major item to look at with this case scenario.

Did you check the Azimuth??

No tubes, although thats my next step, i use a cambridge A1 through a 640p and i have checked and re checked all leads and connections, the azimuth looks fine but as i have the sumiko headshell i can at least play around with that, it's not that it's quite on the left channel as some records come out louder. Joni mitcell's hissing of the summer lawn, steely dan's royal scam and utopias oops wrong planet. i have swapped the phono leads from the deck over but that just changes channels, i suppose what i need to do is swap the connections on the cartridge to hear the mix change say on led zep 2 when the drums come out of the left channel and the guitar kicks in on the right after the the middle section, you know the swirley stereo wierd section. if it's clear then it just could be the way it's recorded, if it's muddy then i will know if there's something wrong. I am really confused.