I can give you a little input of what music I think is better suited on a few different models of Infinity's. I don't have any of the small models of Infinity's other than one pair of QLS-5's which I have very little listening time on. But on the larger models I can give you my opinion. Kappa 9's are pretty good with rock music, pop, and heavy metal. QLS-1's have great bass, mids, and a little more refined on the high's, just as the QLS-2's. Don't get me wrong they sound very good with the rock and heavy metal, but they shine on classical, strings, and vocals. I really like their sound with violins and cellos, and solo female artist, including a quite a few pop recordings. I love the way they reproduce Aliason Krauss, since you mention her name, and they are very good with the likes of enya, Loreen Mckennett, Shakira, and most female artist. They are really good with opera as well, Renee Flemming, Anna Netrebko, Angela Gheorghiu, to name a few. And they really shine with violins and female voices, Lucia Micarelli, Vanessa Mae, Ikuko Kawai, AVassilis Tsabropoulos, Kim Kashkashian, the last being two cello favorites of mine. Some heavy metal is good as well, mainly the Finns and Norway bands of Sirenia, Within Temptation, Leaving Eyes, and nothing better than the crossover of pop and opera of Shara Brightman.
The RS-1b's sound great with all types IMO as they can really play loud music great and still play soloist good, but I would say they are a little less good at playing quite solo music as well. They still shine with strings, brass, and percussion instruments, but they really excell with powerful passages. They can reproduce a very large soundstage and they can play very loud passages so good it is sometimes scary, I can only imagine a pair of Betta's or IRS.
As far as older rock which I grew up with, I find it somewhat lacking, IMO older recording lack complexity and details that more complex recording mixing of new music. I know some people might argue this matter, but when I listen to my older recordings on records of groups like Led Zepplin, Jetho Tull, Pink Floyd, Allman Brothers, Beatles, Doors, Bob Dylan, and lots of Classical recordings they just seem to lack so much over newer recordings.