Weren't the Concert Sound speakers from Bose 8-10 ohm?
Skar is good quality as are the others mentioned, but car audio seems to be as polarizing as politics at times so you'll get a multitude of passionate answers. I outfitted my '04 single cab Sierra with NVX and Rockville and it's exactly what I wanted. NVX is the house brand of Sonic Electronics, and here's a link to their available 3.5" speakers. I don't know if they'll fit under the G Body dash grilles as I've yet to do a complete audio job to my Elk, but I've got their woofers, mids, and tweets in my truck doors as well as replaced the factory speakers in the cab corners.
https://www.sonicelectronix.com/search?keyword=3.5 They sound very crisp and clear, and the bass is there if you want it. I have tinnitus and use music to drown it out, and as a huge fan of music I can't stand bad speakers.
Another brand I'm fond of is Dayton Audio, and they come from Parts Express. I've got their home audio speakers and they're fantastic- they do make car speakers, just not a huge variety of them.
If you have the ability, putting together a component system will always sound better than using 3 or 4 way speakers. Subwoofers/ woofers can't replicate high frequency, tweeters can't do lows, and mids can have a wide range but should really only be used for mid-range audio and not the extremes of its potential. Stacking multiple drivers that rob power and quality from each other into a single basket is not the right answer, it's just the quick answer. They sound "fine" to most, in most situations. Until you hear a component system. The right answer is having dedicated drivers for lows, mids, and highs, respectively and then utilizing crossovers to limit which frequencies you're asking the drivers to replicate. I stuffed some 6.5" Pioneers in the footwells and Pyle Driver Pro Titanium tweeters in my El Camino dash for now but they're not discreet. The Pyles are also harsh and could use a 1 watt in line resistor to calm them down a bit. Save your ears and stick with silk dome tweeters over titanium when putting them into a small cabin vehicle.
Pay close attention to the wattage as well as the resistance of the entire setup- you can manipulate these in different ways for different scenarios, or you can fry your gear if you mix incompatible parts. Acoustics play a major role as well, and Bose has always been known for having high quality factory car audio systems, in part due to their chosen speaker placement. You don't need to break the bank as long as you get a few key things right. Wattage, ohms, speaker placement, and quality wire connections should be at the top of that list. The best speakers in the world will sound horrible if you get those wrong.
SonicElectronics.com, PartsExpress.com, and DelCity.net will have everything you need. I also very highly recommend a mini-amp, such as the Soundstream ST4.1200D. This will require better speakers than what would suffice for the factory stereo, but the ones discussed are fine as long as the wattage isn't exceeded.
Perhaps you could use some Concert Sound speakers for now to get the placement right while meeting the 8-10 ohm requirement, and upgrade to better speakers if/when you change your head unit. Or the factory GM Monsoon system isn't too bad. In fact, I plan on saving the entire setup- from my friend's 2002 Grand Am.
