When I thought I had a voltage problem it turned out that I actually had a ground problem, I wasn't earthing the outer of the phono leads at the amp end. Sorry if you've already done that, but I thought I'd mention it in case you're trying to solve the wrong problem as I was.

Anyway, if it really is a voltage level problem then you could try a voltage divider. I put one together in half an hour by cutting up an old pair of phono extension leads and wiring in a 10k linear dual-gang pot as a voltage divider. This allowed me to vary the voltage of the signal going from Empeg to head unit anywhere between 0 and the full output of the Empeg without significantly changing the impedance that the Empeg expected to feed or the head unit expected to be fed from. It didn't cure the problem in my car (as I was trying to solve the wrong problem) but it vary the voltage/volume exactly as expected when I connected it between my Empeg and my hifi in the house.

Please don't do any of this unless you're sure you know what you're doing as it may be possible to damage your Empeg if you get it wrong. Shout up if you need any help with the circuit. In a nutshell though, image that each pot has a top, bottom and middle connector. Connect the bottom to the outer of the phone from the Empeg and the outer of the phono to the head unit. Connect the top to the inner of the phono from the Empeg. Connect the middle to the inner of the phono to the head unit. Be careful not to get the wiring wrong or you could short the output of the Empeg, though I'm fairly sure I accidentally did that for a few seconds on mine and it didn't seem to do any harm.

Assuming that the voltage divider does the trick, you can make a much smaller and cheaper version by using fixed resistors rather than the pots if you have some means of measuring the values that exist between bottom and middle, and top and middle, when the pot is set up right for your system.

Cheers,

Mark.