In a previous system, I had a set of av 123 Strata Mini's, which had a sensitivity of 86 dB. There was no "matching" CC, (although MLS told me and others on several occasions that there eventually would be
), so I used a Klipsch RC-7 with them, plus a set of RS-7's as surrounds. The Klipsch speakers were high sensitivity, (98 dB), so the difference was very close to what you are asking about. I used the speaker trims to set the relative levels, but I was very close to running out of range with the 12 dB variance of sensitivities.
In addition, it seemed that the sensitivities were not "linear". As the volume went up, the Klipsch's seemed to get louder than the Mini's. Also, I blew the tweeters in the Mini's twice because they just couldn't keep up with, (or possibly didn't have enough amplifier headroom for), the Klipsch's.
After that experience, I vowed I would never mix low sensitivity and high sensitivity speakers in a system again. I will try to keep the sensitivities of all the speakers in a system within 3 dB of each other.
My $0.02.
Craig
Edit: Just re-read your OP. If you were not talking about using the 83 and 96 dB sensitivity speaker *together*, then ignore the above. The difference between 83 and 96 dB sensitivity speakers is how much amplifier power you need to make them produce the same sound pressure level. The 83 dB speakers will require *huge* amounts of amplifier power, whereas the 96 dB speakers will have the amps just loafing along at the same SPL. Try this calculator to explore the differences:
http://myhometheater.homestead.com/splcalculator.html