One of the most subjective threads on this forum for sure. All I can contribute are my comparisons to the current and past cars I've owned with stock sound systems, and as a studio recording engineer as one of my jobs.
One big factor people don't take in to consideration is how well acoustically reinforced their cars are for both exterior and interior noises. People may listen for outside road noises, but the same consideration should be made for how well sound is absorbed inside as well. This is usually one area that distinguishes higher end cars from more affordable ones.
Another factor is how the audio companies work with the car makers to tune their speaker frequencies to accommodate for how each specific car model sounds acoustically.
With that said, the stock Harman Kardon system in my 3.6 model are honestly quite good, IMO. It definitely helps that there is a small and reasonably sized subwoofer in the back along with decent tweeters and a center speaker up front. The sound is less muddy than Bose Systems (over-rated name brand) that I have heard and there is a clarity to the HK system that as mentioned, is partially aided by how the car is acoustically reinforced for exterior/interior noises. I'm not saying the Outback is the best with sound reinforcement, but it's decent enough. That factor probably makes one of the biggest differences in how speakers sound, just like in any setting.
The HK in my ears are better than the premium Bose system in the Mazda 3 we also have (now that is an underpowered, muddy sounding system), better than the stock system in the 2002 4Runner Sport Edition (traded this for the Outback, and I do miss that car a lot), and better than my old late 90's Jeep Grand Cherokee's Limited system.