To close the loop, I replaced the amp with a Kenwood Excelon X802-5. It's a 5 channel with 75W to the front and rear and more to the series-wired door speakers.
It's as good as I can expect with the quality of the speakers in the car (and my failing ears), so I'm happy.
I am however considering replacing the cheap Hertz coaxials in the front with component Focals (ES 100 K). But given that's over a thousand $ and a bit of jiggerypokery to get them installed, it might take a while...
