I chose to leave my stocks and have a Pioneer 860(gotta have a CD player with EQ) and it made my 10 year old speakers sound better than my friends 07 Silverado speakers with stock radio. I'm still amazed by the sound, although it is a bit distorted at times and especially with the radio. But with 2 15"s fired up in the back, you cant hear the distorion, just the highs and lil bit of mids.
I have 6.5" comp set from CDT that I will install sooner or later and run a 100x4rms amp.
My point, if you plan on upgrading the main unit, do this first before spending money on speakers