I agree with a lot of what you said and accept that my perspective is subjective as you point out most opinions on stereo quality are. I am not a hifi hobbyist of any description, simply a music consumer
so, from my perspective
Bose system in the 156 is much better than the standard system in the 159. it's true that there's more power and ofc the bass is deeper, but the sound is also broader, there's more detail, especially at low volumes (and that's coming for a diesel 156 to a petrol 159 so cabin noise is actually reduced to the advantage of the basic system I have in the 159). I have not heard the Bose hifi in the 159 for a real direct comparison and am not sure of the system has been up/down graded since it was moved from the 156-159. I think to say it makes little difference is just pain wrong, but as I said, that is my perspective.
Martin, note that no-one with a BOSE hifi in their car has yet said the sound is any measure of horrible. See for yourself when it turns up but I suspect you'll be more than satisfied
meanwhile, none of you cleverclogs have given totters or I any guidance on what setup would be good to upgrade a standard stereo fitted 159. if you think the Bose is underwhelming, what would be a better use of 750 euros on car stereo upgrade?
bomberesque, I have upgraded a E34 BMW 5-series a decade ago. The installer mentioned to me what should be done to improve the sound, in the priority order (important for people who do not have all the cash or time available at once), so:
1) the head. Check out the options which car radio/CD/DVD head unit will fit your car, wallet and needs. For a budget audio upgrade the amplification may be done only by the head unit. Of course, don't be fooled by all that marketing bla-bla-bla PMPO megawatts vendors advertise. Use DIN standard measurement data instead or other realistic references.
2) upgrade your tweeters and speakers with higher quality ones. I used Infinity Kappa series at the time and was very satisfied with them. Here important to understand that upgrading signal cabling will also further improve the matter
3) more serious (and expensive) setup includes tweeters' and speakers' sitting places upgrade and sound isolation all over the car body -- you will need quality podiums for your sound emitting membranes. Think here about custom-made speaker podiums for doors, pillars and, if you have speakers there, upgrading your rear shelf. Usually, even in premium segment cars, door speakers are installed on the ****ty plastic components. Standard speaker quality is also usually crap.
4) bring in the sub(s)! usually this also involves installing amplification hardware, the best way (and more expensive) is to have a dedicated amp or 2 for your sub(s) and another multichannel amp for all your other speakers/tweeters. Things get oh so expensive at this point. On the other hand, for many people the sub is not necessary in the car, as a quality speakers/tweeters system with good head unit may be enough for some people. A properly designed and placed sub-enclosure of adequate volume will make you heard 2 blocks ahead when you approach, if you need it
But my experience tells that such extreme systems have nothing to do with LISTENING TO MUSIC for long time without fatigue on your ears. At least for me.
Of course even the best components may ruin the installation if done by a poor specialist. Installer has to spend some time croaching all over your car, moving speakers here/there to study the specifics of your interior and fixing acoustical problems. Good setup is largely demonstrated by a very pleasant and attractive sound field, not only clear sound and power. Ideally, all the system shall be planned in advance and components picked to match each other. In most cases, if you complete such a project, your in-car entertainment will sound miles better than BOSE system I auditioned in 159. And the budget may easily stay within EUR1-2K range.
Hope this helps you to make a proper decision.