I would summarily dismiss everything the dealer said as BS, except that bit about Axioms not being good for heavy metal. It seems like many of the new rock/pop CDs are indeed engineered for play on car stereos and come out hard and brittle through the Axioms. Crystal clear, but hard.
'70's metal sounds awesome though. The Axioms open up those quiet recordings beautifully. Anybody listen to Robin Trower's Bridge of Sighs CD? Freakin' great!