Circa 2001 I bought a new Porsche, and wanted to upgrade the stock audio system.
Like any self respecting Audiogoner, I would normally get obsessively involved but was busy at the time and therefore deferred to a "high end" car audio guy in CT who recommended new speakers and amps, while keeping the stock head unit which he thought was very good.
Well a couple of thousand bucks later, it seems that the stock head unit did not even have line level out to the amps! So it all sounded absolutely terrible to me, and not even close to my 1978 Nakamichi a/d/s system - I really couldn't believe how bad it was.
For an incremental improvement, I had another dealer install a newer Nakamichi head unit, which was fairly simple, CD+radio, no pimp my ride lights, EQs or DSP - I believe it was a 2002 Nakamichi CD 400 which appears to be available online for < $200.
The simple presence of line level out to the power amps (duh!) made a huge difference.
So could be worth a look, subject to whatever else is available (or not available) from the peddlars of subwoofers and neon.
Others have recommended Butler tube amps, and Dynaudio (?) speakers. There was also a Milbert tube amp, but I never heard one and they were very expensive.