Do you know the phono gain via your MM inputs? It is (remotely) possible that the gain is marginal for a 2.2mV cartridge output, I suppose. But it would help to have some more details regarding your phono stage; does it have separate MM and MC inputs, as Czarivey suggests?
Other than that, I think you may have no problem; in my experience some cartridges do tend to submerge the vocalist in favor of the instrumentalists, compared to others. This can actually be a virtue of the cartridge, in the case where you are not listening to a single, stage-center vocal LP. For example, I found that my Stanton 980LZS tended to paint that sort of picture. But the Stanton excels at bringing forth internal musical threads within a group of instruments. It's certainly not broken. The test for this explanation, of course, is to try another cartridge.
Nothing that you wrote suggests to me that the two channels are necessarily out of phase with each other, but be sure of that, too. Nothing at all suggests a problem with turntable speed stability; I don't know where that came from.
Other than that, I think you may have no problem; in my experience some cartridges do tend to submerge the vocalist in favor of the instrumentalists, compared to others. This can actually be a virtue of the cartridge, in the case where you are not listening to a single, stage-center vocal LP. For example, I found that my Stanton 980LZS tended to paint that sort of picture. But the Stanton excels at bringing forth internal musical threads within a group of instruments. It's certainly not broken. The test for this explanation, of course, is to try another cartridge.
Nothing that you wrote suggests to me that the two channels are necessarily out of phase with each other, but be sure of that, too. Nothing at all suggests a problem with turntable speed stability; I don't know where that came from.
