my sl 1210 is louder on the right channel

having checked alignment, vta, bias, tracking force and everything else i just feel underwhelmed at it's performance it's not as clear on the left channel although sometimes there's more on the left but it just seems a bit muddy to me could it be the often quoted bad quality phono leads or should i just take it back to the shop for an exchange as i really want to keep this deck but it is peeing me off at the moment. Thanks for your help, Engine.
Try switching the cartridge leads left to right where they connect to the cartridge. If the channel imbalance switches channels, then you have a faulty cartridge (or something off in the cartridge set-up...perhaps azimuth).

If the channel imbalance doesn't follow the switch, then put the cartridge leads back to normal, and then switch your phono interconnects at the table by putting the left output into the right jack and vice versa. If the channel imbalance follows the swap, then the problem is in the tonearm. If the channel imbalance does not follow the swap, then the problem is downstream of the table.

Keep doing this process of switching one end of each successive interconnect pair (first normalizing the upstream interconnects) until you hear the channel imbalance switch channels. When it happens, you'll have found the component that's causing the problem.

Good luck.
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.
Undertow makes a good point. Once you've done the interconnect switching method to track down the offending component, then you can start tube swapping.

Of course, you could start tube swapping from the beginning, and you might get lucky.
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.
Swapping the phono leads and interconnects will change the left channel to the right speaker and vice versa. But if one channel is always lower than the other...say the left is lower than the right, then swapping the wires at some point in the signal chain should not only change channels from right to left, but the low channel should also change from left to right (or vice versa).

If you swapped phono leads, and the channels changed, but the low channel did not follow the swap, then you know the turntable is not the problem (including the cartridge leads).

Keep swapping until you hear the low channel shift.

Do you have this low channel issue with another source...say a CD player?