I redid the stereo system in my Subaru this summer, and here's my take on things after having it installed for ~3 months.
As a note, I went with an Alpine CDE-153BT head unit and PowerPack amp, Infinity Reference 6500cx speakers in front, and Reference 6502ix speakers in the rear.
First, with regard to the PowerPack amplifier: It puts out 4x45W RMS, and conveniently just plugs in between the Alpine headunit and wiring harness. The ease of integration is a wonderful feature. They do also sell a universal one, but that would require significantly more wiring work. My speakers are rated at 5-90W RMS in front and 5-60W RMW rear. Peak levels are 270W front and 180W rear. The 45W RMS is
more than enough. I don't listen above a volume level at around 12 (15 with all the windows open, and rare occasions up to 18 if it's a good classic rock song), whereas the volume on the head unit goes up to ~40 IIRC. I've come nowhere near using it's full potential, and I never will. For the price, it's a great solution.
Infinity speakers: They're AMAZING. I fell in love with them just listening to random sets of speakers at CarToys. They're pricier than a lot of options, but there's certainly more expensive options. My front speakers are components and do have a noticeably better (imo) sound quality than the rear speakers. However, they're worth double ($250 vs $120) is much more of a personal decision. I'm happy with my decision. The best way to find out what speakers you like is to go and listen to them. Do yourself a favor though,
set a budget before you go in and don't go outside of it. I didn't really set a budget for myself when I went in. I listened to the $250 speakers never intending to spend that much on just a set of speakers, yet I walked out of the store with them. You're all but guaranteed to increase sound quality as you go up in price, so if you don't set a budget/limit, you're gonna have a bad time.
As for installation, it's not a terribly difficult task to accomplish yourself. To get to the innards of my doors, I had to undo 3 screws and then the door panel pops off. Speakers are held in with 3 screws, and wiring adapters are readily available to convert from the factory connector to bare wires or the proper connectors to attach to aftermarket speakers. Amplifier installation is a much bigger PITA and require running wiring from the engine bay (or is the Miata battery in the trunk?) to the amplifier, typically mounted in the trunk. You also need to run wiring from the head unit to the amp, and then from the amp to speakers (this is my understanding, my only experience with what I'd call a more traditional amp was when I had a subwoofer, and that's slightly different). Something like the Alpine PowerPack solution is essentially plug and play. Additionally, head unit install is also fairly simple. Dash kits (if required) are fairly simple/straightforward to install. The hardest part would be connecting the factory harness to the stereo, and there are companies that sell adapters to convert from the factory harness to bare wires that you'd then connect to the connector included with the head unit. If you can match colors of wires, you're all set. I use posi-lock connectors (available on crutchfield) to take care of wiring connections rather than soldering (so it's more easily removable/undone later if necessary). I've never had a problem with them.
Lastly, for retailers, check out CarToys for selection. I've never had a problem with them when I've gone in and they are fairly knowledgeable. I know there are a few other chain-type places that I don't really have experience with.
As for an installer, I'd actually recommend PDA Road Gear (
http://pdaroadgear.com). I discovered them at the end of the summer looking for an Escort Live DirectWire Smartcord for my radar detector, along with a mirror mount. They were able to order both and get them shipped out on a Friday afternoon for me to pick up Monday. They do have a fair selection of stereo equipment worth checking out as well. I didn't have much time to look at that, and I didn't discover them until after I had purchased and installed all my equipment. I obviously can't speak to the install quality, but my experience is more than enough for me to want to recommend them. They had an NSX and a 20s/30s era hot rod Ford in for installs when I was there. They were wonderful and I highly recommend them.
See, I'm working on my engineering degree, so I worry about what's cool not what's reasonable! lol.