The quality of the source does matter, however, the upgraded audio system I installed in my Forester performed far better with all sources whereas the Ascent's system does not. And it's difficult to provide excellent source material for everything you want to listen to.This also reflects my experience...quality of the source matters.
I have been able to improve the sound of the Ascent's HK audio system with excellent source material, but it still falls far short of my expectation for a high-performance system. Better source material considerably improves the bass performance, but without a suitable equalizer, the sound to my hears remains much too harsh, especially in the mid-range. Even with the best source material, I find it painful to listen to for any length of time.
However, audio performance is highly subjective, some may find the performance acceptable whereas others may not.