which tubes for which genre ?

15465550, the Audience 42 is built for transistor amps in an attempt to get more power into the speaker without affecting its apparent sensitivity. But the speaker can be driven successfully with tubes, and while you will need some power, you do not need a power house amplifier- much more than 150 watts and you can damage the speaker.

I would recommend something that has at least 60 watts or so. Some tube amps will drive the speaker very well directly, but IME many tube amps do have troubles with 4 ohms, so my recommendation is to also consider using a set of ZEROs (see http://www.zeroimpedance.com).

If you do this properly, you will find the bass of a good tube amp to be actually better than that of most transistor amps.

Just before Ralph (Atmasphere) chimed in I was in the process of recommending his amps (Atma-sphere) for the Dyns and those that want to rock with tubes. The 60 watts the M60 pushes with the zeros to raise impedance would do justice to the Audience 42's and most dynamic driver speakers. Despite Ralph's comments I would say add a good sub because of the small woofers in the 42's and rock the joint.

I am driving ACI Jaguar 2000's with M60 v.III amps with great satisfaction. The Jaguars have Dynaudio drivers but the Atma-sphere amps have never blinked. Bass below 70hZ is handled by an ACI Titan sub.

Entrope (the other Ralph)