Well, I took the plunge and fitted the sony XSF1731s to my 306.

I was pleased that they fitted straight into the doors with no cutting
etc.

(For those of you interested the original speakers have a mounting
depth of 62.5mm)

All was well until I turned the volume up! Crackle or what!
The original speakers are 25w rms, the new speakers are rated at 30w.
(180w peak, my ar$e!)

The new ones are better and bass-ier until the top 1/4 of the volume
range when they are awful,
even with just my 50w per channel head unit.

I'm gonna have to get some better ones, any suggestions?
16.5 cm diam, max 65mm mounting depth and able to handle a 50w/channel
head unit?
I didn't think I was asking too much!

R.