Try searching for a thread called 'reduced sound out put after replacing o/e bose ?' Not sure how to do a link to it I'm afraid!
Basically I had the same when I replaced my Bose head unit and it made it very quiet with not great sound quality. I then used line boosters to up the volume which made it Better but still not great...
And then I found this thread, did the wiring mod which cost all of about 3 quid and it has transformed my stereo into something very respectable! Very loud volume, crisp sound, good bass..
So I would recommend trying this first before throwing loads of money at it :-)