In the interest of improving my audio in the 14 hardtop with HK I did the Bavsound stage 1 speakers. It helped but not earth shattering results. Instead of further amps and speakers I purchased a Fiio X3 digital audio player and am using that instead of a USB stick. Wow, night and day difference, much fuller sound even at lower volumes.

Only downside is it is harder to just change selections while driving.