I want to thank everybody for their input and recommendations. I weighed them all, along with my own requirements and some other input. I ended up going with a pair of Rockford Fosgate R1693 "Prime Series" from Crutchfield.
For a quick, cheap and easy
improvement over something that didn't deserve the name "Concert Sound II", this package ain't bad. Some of the other choices available elsewhere either didn't fit according to Crutchfield's fraction-of-an-inch database, or else would have required about $17 more to order the GM adapter cables a la carte from Crutchfield, that were included for free.
I'm not an audiophile, I simply wanted something more than weak bass to come out of the rears. These new speakers have more capacity than I needed, or could "light up" with the factory system: 60Watts nominal/120Watts peak, and from my best guess, about 30 ounce magnets. Strangely, nowhere could I find magnet size specified, but since they're at least 3 times the size of the factory units, and considering that the 2 speakers with packaging, screws and wires weighed in at over 10 pounds, I figure they must
be at least 30 ounce magnets...
Before replacement, I turned the fade all the way to the rear to listen to the original speakers one last time. As the frequency response of a telephone line is about 3 KHz, I'd say that the factory speakers weren't putting out anything above 2 KHz... (As a side note, these factory units in my '97 were
still 10 Ohms.) Also, I'd had the fade biased to the rear just so I could hear something
from the back.
Once armed with a T15 Torxdriver, a flashlight, a couple Percocet and those custom GM adapter harnesses, it literally took only 20 minutes to bend myself into the trunk and replace both speakers. It is truly idiot-proof because the adapters have polarized spade connectors to match the speaker connectors, and the database knew that another pair of Blaupunkts that I was considering had tweeter/midrange speakers a fraction of an inch too tall to fit under the package tray upholstery. In fact, fitment was such a near thing with the way that the upholstery "domes" over the speaker cutout, that I had to mount them so that the larger midrange speaker is on the side with more clearance.
After extricating myself from the trunk, I fired up the radio and CD to listen to them and readjust my settings. By comparison, I do have to run a little higher volume to feed those larger, lower-impedance magnets, and I'm a little disappointed that the midrange & tweeter still don't match the treble of the front speakers. An amp would probably improve both.
However, after going for a short drive and fiddling with volume, fader and equalizer, the difference is unmistakable. If the old speakers (through upholstery and DashPad cover) were putting out 2 KHz, then these are putting out 8 or 10... Also, the balance of power now works with the fader in the center detent, so I'm getting more relative treble via the front speakers as well.
A half-hour and $77 for everything including shipping... I'm content