Based on my experience... The Audison equipment is better than any of the items on Crutchfield or a Line-Output Converter. Here's why:
It takes any signal...Digital or Analog...converts it to Digital...allows full Digital Signal Processing for Phase/Time Sequencing/Distance Matching/Frequency Splitting/the list goes on... and then sends that signal out to your amp. They're not cheap...and professional install is an exceptional idea...but the sound quality is amazing. The integration, if done right, is flawless.
Nothing else I've "heard" sounds this good.
Picture one of the crappiest sounding stock stereos... It's the Ford F-250 Crew Cab XLT (think 1 notch up from the worst options package ever).
An audison Bit 10 D, 4 Coaxial speakers, 1 JL box (not even a big one) and a JL 700/5 amp. Sounded like the concert hall I spent $$$$ on to make my BMW have proper staging/sound depth and clarity. For significantly less.
(disclosure...i'm not an installer...not a dealer...just make airplanes fly...)