In this post, I am combining my experiences upgrading my in-car sounds..
I am not an expert, but to me it is a bit like wine & art - bugger the experts, I just like what I like.

I bought my Daimler 3.2 AJ16 5 months ago.
The cassette sounded tinny, I had no tapes left, and the CD changer wouldn't connect 90 % of the time (sound familiar?)

So..

Stage1:
I needed good hands-free phone for work: I chose Parrot Asteroid Classic for low-key look to suit the car, and good hands-free phone & music selection. The Parrot Installer upgraded to JVC 6.5" speakers. He failed to 'upsell' me - I was in his shop wanting to buy Alpine etc, but he recommended JVC to save me money.
All good for a day, then one speaker started to 'buzz' on the door card - back Ol'Blue went, and it took a day to suss the problem and swap in some 5 1/4" JVCs. Problem solved, in fact a bit more bass than the 6.5s - go figure

NZ has coarse-chip seal on the roads, so lots of tyre noise, and I wanted some Bass, so..

Stage 2:
same installer, again, didn't upsell me - fitted a Sony 600w amp, and smallish Sony sealed sub - this suited me as I wanted some trunk space to remain. The sub is mounted on wing nuts using the old CD changer bracket, so I can remove it if I need the spare wheel of a dark & stormy night.
Result - reasonable bass, but a mid-bass 'hole' when driving, which I attributed to the coarse-chip road noise.
So I wanted some rear parcel shelf speakers running of the trunk-mounted amp, to fill in the bass hole.
Meanwhile, lots of delayed maintenance with my favourite mechanic over 3 months - new front suspension bushes, drive shaft hanger bearing, water hoses, front shocks replaced with KYBs which I didn't like so original Bilsteins rebuilt and refitted. All this work delayed the parcel shelf speakers, plus I didn't want to destroy the original rear shelf with its pristine Daimler blind, so I bought a plain shelf off a wrecked XJ6 from Beacham Jaguar (no blind) to cut holes in for the speakers...
Meanwhile, an insidiously-increasing rear rumble traced to the diff bearings last week, and new diff bearings fitted. Very quiet!..and you can hear the stereo now!
but the installer had bought in my Alpine R-spec 6 1/2" speakers to fit in the shelf, so...

Stage 3:
amp'ed Alpine 300W 6 1/2" R-spec speakers fitted in the parcel shelf, awesome now the diff is quiet. Keith Richards never sounded so good on 'Can't You Hear Me Knocking' at 120 km/h.

I am finally happy.

However various experts here (Jake21 in XJS 'stereo' thread) and a local Napier audio guy here in NZ have told me the Sony amp & sub are pretty crap, to paraphrase (Sony's "600W" apparently about 200W, for instance).
At the moment, I am happy and not spending any more - the next step would be a better Alpine amp in the trunk, a better sub, and Alpine type-R 5 1/4s in the front doors. Maybe in a year or two.

If I knew 6 months ago what I know now, I would have got either the Parrot again, or a top-line Pioneer I have seen, a better amp & sub, high-spec Alpine door speakers, and done it all once not 3 times.
The Parrot, in all honesty, has driven me mental at times. It worked for a month then started crashing and freezing. I was ready to ditch it for the Pioneer at that stage. However a software upgrade via the internet sorted it (from v1.6.1 to v2.0.0), and it runs far more stably with all my music on a hard-drive running through the USB than through the iPhone.
I would recommend the Parrot, but get a hard-drive too, and upgrade the software via a Windows-based computer - it didn't download properly on our household's Macs...

..my 2 cents worth.

(pics - Parrot Asteroid, sub & amp, rear speakers, old buggered factory speakers which spent 10 years in Singapore before NZ)

 

did they open up the parcel shelf into the trunk to help those rear speakers out? Never mind - I just read your post again.

Can you post what speaker models you used?

 

wow at the stock speakers condition. when i swapped mine they where still in excellent condition. i like those alpine components in the rear deck.

i agree with what the person told you about sony. but that doesnt mean you have to swap it. if you like how it sounds thats all that matters.