Left channel much louder than right?

It is probably due to improper compensating forces to offset the cartridges natural desire to drift to the outside of the groove. As I recall your arm doesn't have built in anti skating compensation but the manufacture does have sugestions for doing something, like twisting the phono leads or something. It is also possible that you would over come much of this by increasing the VTF to the manufacturer's max spec.

If the azimuth of the cartridge is off this can happen too. Basically what I mean is that the cartridge is not sitting perfectly perpendicular to the LP, it's tilted to one side or the other.

I'm not familiar at with the JMW9 arm - but I'm sure there's a way to adjust it. I'm sure someone out there will chime in with the necessary instructions and to correct me on correct terminology for this setting - I just can't recall what it is other than to call it the azimuth setting.