well, the best idea seems to be to set up with the empeg at 0db (or lower if there is clipping) to give a decent signal to the amp.

Then just change the amp gain to suit.

Slight adjustments are likely to involve reducing the empeg gain a bit, bringing the amp up a bit, so that when you are on the motorway you can increase the volume.

I tend to have mine at -18db for city and -6db for motorway. I take it up to 0 on some noisy roads. I also have voladj set to medium. This means my amp sits at about 3.