Accuphase or Leben for heavy metal

Extremely underpowered amps. Metal has a lot of energy so the average power consumption requirement is rather high. Add in transient peaks and you can see the problem. Metalheads don't listen at 90dbA very often (or 100dbA for that matter) so having extra in the tank of the amp will help. The metal genre is actually very demanding on a system. Some call speed of their speakers the ability to reproduce the subtleness of some recordings, I challenge them to try to reproduce some metal songs and keep it's clarity throughout. 

I would consider class A/B to get the power output up to near the 250W/ch 8Ohm range but with a ton of stiffening capacitance and large toroidal transformer. Metal sure can sound compressed as the volume rises and with an amp with a ton of power in reserve will elevate that. 

I agree with @piebaldpython  that you should go A/B and higher wattage. Look for an amp that specifies a certain amount of Class A watts, this will give you good sound for lower volume listening. 

I agree with those above.

The Leben sounds like a simply elegant amp I would love to own and play string quartets and traditional jazz.

Neither, Accuphase or Leben for heavy metal. I suggest SPL from Germany. Their demo at AXPONA was impressive on all genre of music. 
 

@apanhc Just so you know, there's no way to design an amplifier (or loudspeaker) for a certain genre of music. As long as you have the bandwidth and power to reproduce the signal at a lifelike level you're golden, as long as the electronics and speakers actually sound like music (rather than electronics) while doing their job.

If you like to play loud, you might consider more efficient speakers. For example if your speakers were 96dB instead, you'd only need 1/4th the amplifier power to make the same sound pressure and quite often you get greater dynamic contrast and greater detail at the same time.

The greater dynamic contrast has to do with less thermal compression in the voice coils that you get with higher efficiency drivers. The greater detail often has to do with the efficiency as well, since the drivers are often faster.