Ok so heres the deal:

I have my Empeg Mk2a plugged into the auxilliary of my Kenwood Z828 head unit. The Kenwood in turn is hooked to a 2 channel Infinity Kappa amp. The Kenwood's volume goes upto 35 and the Empegs to +10 Overdrive or something like that.

Well anyhow, if i have the volume on the Kenwood at say, 22, and the volume on the Empeg at -13.5 dB, everything sounds nice and clean. But say i turn the volume on the Kenwood down to like, 12, and turn the Empeg's volume up, say to 0 dB or something (anything really), it'll sound like crap --just distorted and dirty. This is with the Eq's on both being set to Flat.

This doesn't really bother me or anything, as i just use the Kenwoods volume as the main control, but i'm wondering why it happens. maybe its a stupid question.

If the Kenwood unit's input stage is only supposed to accept 1v inputs instead of 4v inputs, that would explain it. By turning the empeg down to -13db, you are doing the correct thing to compensate for plugging its 4v outputs into the Kenwood's 1v inputs.

Speaking of which, does anyone know what the volume setting should be to ``precisely'' compensate for 1V inputs?

I think every line output is going to be a little different in level, and every line linput is going to be a little different in the level they'll accept before distortion. The 1v and 4v things are just an approximation.