Jeff,
sounds like you have the JVC 850 or similar head unit in your car?

I have the JVC850 head unit with remote control (actually I have 2 - one in my wifes car for her empeg player and one in mine for mine!) and both work fine with my empeg line-input volume wise.

Some JVC head units (mine included) allow you to 'adjust' the Line-input level on your Line-in connector between 0 and 5 - read your JVC manual that came with the unit - it explains how to do this. Level 0 is quite attenuated compared to Level 5 and this may be your problem here.

My unit has the line input level set to 2 or 3 and the empeg volume is normally on -3db or lower to get comparable volume to the JVCs Radio - the exact volume level depends on the MP3 being played.

If you have a JVC 850 (or similar) head unit like mine then the JVC remote control for it comes with a bunch of buttons on it to control the 'Line input' device and includes buttons for FF,Play,Stop,Rewind,Pause etc.

You have to be aware though that these buttons are intended only for control of the JVC VCR you can connect to your head unit [there is a wired control cable that you plug into your JVC VCR that allows your JVC remote to control it].

And these buttons only work do something when the unit is in Line input [aka VCR] mode and no other time.

Unfortunately while these buttons on the remote [and the preset-tuner buttons on the head unit when its in line input mode] produce a output on the 'control signal' connector on the back of the head unit, its a JVC specific command format and not empeg compatible or directly usable.

I did toy with the idea [and wrote a small microcontroller program to dump the signals from this control cable to investigate further] of capturing these buttons from the JVC and then feeding them to the empeg - either via IR or via some wired interface [like the forthcoming Sony RMX4S interface that comes in the box with the empeg tuner module].

I planned to do the interface using a little microcontroller but in the end I found I hardly use the JVC remote anymore, and the empeg one is good enough and 'better sized' for use in a moving vehicle - oddly enough as it may seem.

But, what I'd really like to do is full control of my JVC head unit from the empeg - e.g. use the Sony Wired remote controller interaface and emit the control signals required to control the empeg as well as the JVC head unit. But thats a long term project.

You could of course buy a learning remote that can learn the empeg signals and the JVC ones, but that will probably be a bulky remote compared to the either the JVC or empeg remotes so for now thats a non-starter IMHO.

I think when the empeg tuner comes out I'll just leave my JVC head unit in line input mode all the time and use the empeg as tuner and mp3 player and be done with it - at least that way I have a unified remote control of tuner and mp3 playback.

Actually what would be really nice is if the JVC had a 'CD (only) line out' signal I could connect to the empeg's AUX IN - that way I would be able to play CDs in my head unit and route the CD signals to my empeg - I could then control everything from the empeg and use the JVC as simply a in-dash amplifier.
This would still let me use it as a head unit whenever the empeg is out of the car.
But thats not an option on my head unit AFAIK.

What I'd really love also is to be able to control the JVC head unit externally [e.g. send stuff like song titles to the display on the JVC unit], but thats also a non-starter as I doubt the JVC will accept commands from the external control bus and right now I have other things to do which preclude me from investigating further.

Anyway, I hope you can get your JVC head unit volume up a bit more - certainly the JVCs I have work fine and I would have no problem recommending them to someone else as a good basic head unit with CD playback.