although they may be louder, i think the change is most importantly the new recording technologies get much more bass in the mix. Listen to Powerslave, then listen to AMOLAD and you will notice that the bass sound (not the instrument, the pitch level) is far more pronounced. This is especially noticeable on rap albums.
"I only got the sunny hours, the brightest hours of day
I never count the gloomy hours, I let them slip away"
-Long Beach Dub Allstars