For you guys looking for dash speaker recommendations I have been planning my future system for awhile now here is my shortlist as of now. Technology changes so fast so tomorrow this all might change.
3" Full range speaker. They may look like lame stock speakers but they perform much better. I have them in another car and are very good. They have high sensitivity so you get more volume per watt. If you are using a volume high enough to hear distortion consider using bass blockers and
foam baffles to create a kind of box. A pair of these could replace the 4x10
Tweeter with wide range/midrange. Certainly has less bass but relatively plays low into the midrange. And handles big power. These are very likely going into my stereo dash locations.
4" coaxial. I know nothing about this brand but in my search for well reviewed high wattage (for my convertible) small speakers this was a nicely priced option. JL make a more expensive 3.5" option
4x6 plate option. Again well reviewed, high sensitivity and high power. Biggest issue is size. Side by side these would be 12" long total so you would have to get creative with a mounting plate into the stock 4x10 opening.
I've had good luck with Infinity Reference line of speakers too. I have used many of these in stock systems. And would like to try JBL GTO line as well.
In regards to bluetooth amps I agree it totally removes the need and expense of changing the head unit. This assumes all your music is on a phone or ipod (with Bluetooth. But what worries me is changing technology and the whatever is built into the amp will not - change. It would be a shame if your new snazzy phone would no longer talk to you expensive amp. So my suggestion is a regular old amp (cheaper) with a external
Bluetooth receiver/adapter like this. I have not used this one but something like ity. Perhaps any of the home Bluetooth receivers would work if you convert 12v to 5v. This way you just need to change BT receivers as tech changes.
There are other
Bluetooth interfaces that are even more flexible and allow for multiple amps and equalization. And something like this would allow a working stock radio to be used in addition to BT by using the high level (speaker level)inputs. I believe there are old fashioned equalizers with Bluetooth built in too. And lastly, when you are looking alternate ways of getting your music other than a traditional head unit, don't be surprised if it brings you to marine or motorcycle stuff.