I thought I might chime in since I have the hi-fi system in my E90 and upgraded to the JL600/6 last year after much reading on here, but kept the stocks speakers.
I was looking for a modest improvement, mostly in the lower range which is the worse part of the hi-fi system IMO. I wanted a little more punch from the underseats and the JL600/6 budget solution gave me just that (with a Technic harness). I went that route since almost everyone agree that the amp is the first weak link to upgrade. From there, if you think you want more power/clarity, you have enough power to drive aftermarket speakers.
Fast forward to today, if I were to do it again, I would consider the Helix DSP before the JL600/6 for flexibility, but it could lack power if you are to add speakers down the road.
Next step is adding MBQuart kit, but I'm not sure if the improvement would be worth it...or if I really need it
