Wood affecting the tone on a solid body electric guitar? Hang on! Let me get my hardhat to the ensuing war of words.
I kid.... that's an inside joke to a recent thread on wood and cutouts affecting tone.
Seriously though. I've owned RG's from all of these eras. I honestly don't recall the tone exactly from my old 1991 RG570 but there are distinct differences to the 2001 RG570 I own today, which IMO is FAR better than the 1991 version. I also own an 06 RG1570 and an RG550 20th.
Comparing the 1991 RG570 and 2001 RG570.
One piece maple neck in the '91 versus the three piece neck with the bubinga stripe.
A slightly thicker neck profile
Plastic pickup rings in the '91 (crap!) versus steel in the 2001.
The old tilt joint square neck joint on the 1991 versus AANJ on the 2001. (this IMO might be the biggest difference tone wise - second being the three piece neck.)
Also I had the exact same bridge and neck pickup in both guitars; a bridge Tone Zone and a neck PAF Pro. The TZ for my setup has a lot of top end clarity, notably on picking, while having lots of bass, which to me gives it that car engine growl.
The RG1570 has direct mount pickups which I prefer over all.
The RG550 is front route so I can hear more "popping" in my picking from how the pickups are mounted. Maybe that's just me but it definitely sounds, to me, different from the other three. The RG550 is also the 20th with the titanium stiffeners and, while the profile is very close in my memory to the 1991 it sounds better to me.
If I had to rank based on sound.
RG570 from 2001.
RG1570 from 2006.
RG550 from 2007.
RG570 from 1991.
I should add I own a stock RG921 which I dig quite a bit but being fixed bridge with Dimarzio/IBZ's and strung with 10's its a bit of a different animal from the ones listed above.