Left channel much louder than right?

You say you know that it's not a tube or interconnect. How do you know it's not the ics?

I encountered a similar problem at the same time I purchased new preamp. Therefore, I swore it was the preamp.

But then after performing a series of tests with the ic's (crossing L&R from source to pre, then from pre to amp, etc), I was able to determine that it was the right channel ic from the source to the pre that was the culprit.

The mfg'erer confirmed the problem and replaced the cables the one pair of cables. Problem fixed.

-IMO

make shure the cart connected properly.

I would suspect the cartridge itelf, or the wires in the arm. Incorrect setup (angles and forces) will make it sound lousy, but, in my experience, have little effect on loudness.

Try swaping channel connections at the cartridge.