Yup, that'* the same effect I'm getting with the Infinity 6x9'* I pulled from my 1998 SSE and put in my 1989 Lesabre. The SSE had the BOSE amp which really was a nice compliment to the Infinity'* which performed like 8" subs and nicely reproduced the highs, but now they merely sound like blown 4" speakers.
Heck I can't even hardly turn the volume up over 10 (max volume is 40) and the Infinity'* start to distort, and that'* with the equalizer set so as to allow for the highest volume without distorting. And what really makes it sick is the original 4x6'* in the front dash actually sound better and have better bass.
I have a slightly better deck (Pioneer DEH-P4400DH) than the 7-band eq CD deck I had in the SSE, but the sound is no where near as loud or clear. In fact it is extremely distorted in the Lesabre (I wonder if it has to do with the factory boxes with vents that the rear speakers mount in)