mate,
i have the same problem....
i have a Pavilion g4 1104dx laptop and my bass is completely GONE.
I plug in my ipod to my external speaker system and its perfect... but through the audio jack its horrid.
i've played around with troubleshooting and loaded the SRS Premium sound file... but it did nothing so i deleted it again (as you also found).
All my levels on my system equalizer seem normal and my itunes eq is normal also...
what is this random problem?????? does anyone know????
I called tech support at HP and they tried to charge me 50 bucks as its out of warranty lol...
thanks