preamp voltage on Kenwood KDC-MP5028?

I don't have personal experience with that deck, but I have noticed that setting gains according to the specs on the deck and amp are never accurate. It's usually higher on the amp than whats suggested on the deck.

You can certainly test the preouts with a meter and test tones. I recall hearing that pre amp voltages fluctuate with varying frequencies.

I'm not an installer and I know you are, and you and I both know that you have worlds more experience with setting gains than I do... but from what I've experienced and heard, I wouldn't take any claim of output voltage from any manufacturer as gospel.  And I wouldn't assume positions of the gain control as indicative of relative voltage input.

My position is:  make a good test CD that works for your gain-setting procedure, and use it on every install.  Same way, every time.  When you hear the clipping you don't even pay attention to the gain position on the amp.  You just know that the units are matched.  If I did this more often I would pick up one of those handheld O-scopes.  PE is giving away a DMM with those now, IIRC.