which is a better match for best results

+1 atmasphere. People usually listen to rock/pop at much louder volumes so a bright system becomes more obvious. Same system may "seem" to sound better with jazz at lower listening volumes but that's just a misconception and becomes clear as soon as you hear both genres on a tonally balanced system.

Ok, not sure what absolutist bible exists that portends the efficacy of all amps for all music types!  THAT is lunacy.  Classical, jazz and rock can favor very different frequencies and musical tastes.  What can make strings and woodwinds sound natural can make cymbals and upper register piano sound dull and rolled off.  Amps and systems in general have different voicings that can either accentuate certain frequencies or not...of course neutrality is the goal (for some), but can be elusive.  Cables are a whole other realm, as well as source material and playback equipment.  It’s called design...hello!!  Not all amps, or anything for that matter, are made exactly the same.  Are you “All amps sound the same” people drinking the Audioholics protein fueled punch?  Perhaps you were speaking to the ideal amp which would not favor any particular music?  In that case, carry on.

Ok, not sure what absolutist bible exists that portends the efficacy of all amps for all music types!  THAT is lunacy.  Classical, jazz and rock can favor very different frequencies and musical tastes.

This may come as a bit of a surprise, but designers don't 'voice' an amplifier to work with a particular genre of music. There literally is no way to do that!! What does happen is some amps are quite literally better than others- lower distortion, wider bandwidth, that sort of thing, and you can hear that.

Those amps that are better will sound better on all types of music. If it happens that someone auditioning two amps where one is simply better, they may come away with the impression that one favors a certain kind of music. But that is simply because they didn't listen to all genres during their audition (which really isn't possible- there being so many) and one must keep in mind that all recordings are flawed- rock , jazz, classical and so on.

If an amp is limited in bandwidth, it will suffer for rock as it does classical, jazz or folk. If the amp makes unpleasant distortions (such as higher ordered harmonics or IMD) it will sound harsh on **all forms** of music.

No lunacy- just science. This really is a persistent myth. The same applies to speakers- for example the JBL L-100 (recently re-issued) was long said to favor rock, but it does just as well on classical, country and jazz. Again, quite literally, there is no way to build a speaker so that it will favor a certain genre. You can boost the highs and lows and maybe that's more pleasing to you for rock, but if you play a different genre you will find that the boosting of the highs and lows serves just as well there- IOW, such colorations are likely working to compensate for colorations elsewhere in the system where this sort of thing is perceived.

atmasphere (Ralph) is spot on.

@erik_squires: Sometimes when someone says something that we consider funny it's sometimes due to our lack of knowledge from which they speak (I'm being kind here). Happens to the best of us.

That's even funnier. 

^^ Erik, perhaps you can enlighten me- how do you tweak a speaker so that it does rock better than any other genre?