I am running an amp (under the passenger seat), a small sub, and four component speakers.
Amp and speakers are alpine, the sub is JL Audio.
My headunits have been very tasteful old school Kenwoods but they haven't kept up with technology, and the modern ones are very..."blingy"... so I have the SQR 46 I will be installing here soon.
So, with the component speakers front and rear deck I have four tweeters and the sound is crisp and the tweeters are somewhat adjustable giving the highs some clarity.
I took a chance installing all component speakers but the sound has been amazing for an old 911...the subwoofer, behind me on the folded down seat (or I can hide on the floorboard), give just enough punch.
I love it and it's loud when I want it...
As I've stated in other stereo threads, I drive my 911 almost daily and do cross country drives in it...my phone works through the stereo and my GPS (Garmin) actually works through the Garmin app and it comes through the stereo also...
The car is over at a storage unit today but some of my setup is:
I rewired everything myself and it really wasn't that bad. All new power wires and speakers wires. Really was an eye opening experience as I got to see the original rats nest of wiring for traditional stereos and faders, etc...
I had zero experience and honestly, if I can do it anyone can.
I have zero regrets and love having a functional stereo in an old 911.
There are some pretty knowledgeable folks here, already posting, take advantage of them...the forums really helped me install my system
Hope this helps a bit -
Erik