Are such peaks normal in tweeters of this caliber, or do I have a defective component somewhere?

As has been pointed out earlier in this post, MB Quarts are notorious for excessively bright tweeters -- that titanium dome, coupled with the inherent efficiecy of the speaker itself, can cause problems. In my previous car I got away with them simply because I had so much mid-bass (6.5" high up in each door, 4" in the top corners of the dash, a 5.25" center channel, plus 6.5" ambient fills back in the roof and a 10" sub running full-range) that I actually needed the brightness to bring my sound stage and imaging up to where they should be. Putting the 6.5" in the doors absolutely killed my sound quality; the MB Quart tweeters brought it back.

My current system (presently under construction in the ShoWagon) will be using silk-dome tweeters because I won't have to compensate for the overwhelming mid-bass in my previous car.

I suppose I could somehow route the head unit's output into the Rio's input and use it's EQ, but that would be a rather cumbersome solution.

It shouldn't be too difficult -- just run your line-outs into the auxilliary inputs of the Rio. Two 6" pieces of coaxial cable and five minutes under the dash is all you need. The advantages would be considerable -- especially having use of the Rio's fully parametric equalizer and outstanding s/n ratio.

Are such peaks normal in tweeters of this caliber

It may not even be the tweeters, but the acoustics of the car itself. Where the tweeters are placed, the amount of reflective surfaces in the car, the total air volume in the car, and dozens of other variables can cause undesired peaks and valleys in your frequency curve. It will be different for every car, which is why a lot of experimentation and tweaking is always necessary to get top quality sound.

tanstaafl.