I guess, what lovelyred159 intended to tell us is that a large part of the price charged for BOSE audio upgrade option for a car or stand-alone system covers their investment into marketing and "penetration into the sales channels and availability", but not into engineering and quality audio equipment manufacturing. And many people here started to advocate BOSE upgrade as if would lovelyred159 mentioned "BOSE upgrade sounds worse than the basic system"
Personally, I'm into professional studio sound (hobby) and my ears heard some gems in terms of audio setup. But of course, i always remember that an average man's aural needs are quite basic and I do not compare here the in-car acoustic system costing 100-200EUR to produce with above-EUR10K mastering studio grade audio monitors.
To my taste, 159 basic audio system sounds like a plastic crap -- I use "plastic" here as a reference to cheap-plastic-speakers-sound with bad sonic landscape and "synthetic" nature. And BOSE upgrade doesn't change much, as the "synthetic" nature of the sound remains, just lows get amplified and some extra power added. Audio perception is highly subjective from human to human, also for one human when he is in his 20-ties and then in his 60-ties. But here's a simple way I propose to evaluate your Alfa audio system: take an audio CD (not mp3 track) of your favorite music, preferably NOT a heavily compressed dance track with auto-tuned vocals, but rather a track with acoustic drums, electric bases, acoustic guitars, piano and NORMAL conventional vocal from people who still CAN sing. It may be a very dynamically reach piece, it will be even better. Get in the car, set ALL EQ, timbres to "0", sub value to a modest amount - it should give lows, but merely noticed, not "in-ya-face" (there's some kind of AudioBoost feature, if I remember correctly, in 159 - pls turn it off, as we're looking to get "clear" sound the system can reproduce). Listen to your track and try to get the sense of the whole experience, be relaxed and close your eyes. Then go to any home hi-fi system with WOODEN speaker cabinets (shelf of floor-based -- the hi-fi systems and amps from 90-ties are a good bet) and listen to the track. Once again, AUDIO is VERY SUBJECTIVE, but I believe there's a good chance you will hear what I mean by "plastic" sound in 159.
As for good examples of factory fitted audio setups I heard recently, I was really impressed and blown away by top of the line Logic 7 system in BMW 7-series MY2004 or 2005.
All in all, it's a matter of your ears, free time and free money. Which way to go depends heavily on your subjective perception of "quality audio" and country of residence. In some countries, like Russia and Latvia for instance, there are many quality aftermarket audio installers and they charge reasonable prices, plus people less care about warranty as significant volume of the cars are out of warranty already
On the contrary, 1-3 year old lease cars + expensive aftermarket audio installation costs may be prohibitive in some countries. So, sometimes a BOSE upgrade is the only thing most people prefer to do.