What is the best HEAVY METAL speaker?

Speakers and electronics really don't care what the signal is. If they are good at their job they will not be genre-specific. Its really that simple- if a speaker is better at one thing than another, its not a very good speaker., quote by Atmasphere.
Damn Ralph, you are a mind reader!, here we go again, I agree with you 100% my friend, happy Listening.

To my ears and body nothing handles the heavy metal genre (as others) like pro drivers, i.e. through compression drivers via horns or waveguides and larger diameter bass/midrange units, preferably in addition with a sub (also with larger diameter, 15"+ units).

My recommendation, also to stay within affordability, would be for the Geddes loudspeaker model Abbey (would have preferred the larger Summa model, but I'm not sure whether they're still made), which are in the need of the assistance of a sub. They're to be had both as DIY and pre-assembled, ranging from $1,700 to $3,000 per piece.

http://www.gedlee.com/loudspeakers.htm

These comprises pro drivers from Italian manufacturer B&C, and what's very important here is how they're able to withstand serious beating without breaking a sweat (many others brands of pro driver would do similarly well), hereby providing that elusive but extremely vital aspect of effortlessness to the sound. Moreover they're able to communicate the edge, energy and sheer feeling of power needed, not only for this genre of music, without in any way sounding harsh.

Many an audiophile would disagree on pro drivers not sounding harsh, but well implemented they'll cleanly give you dynamics (both micro and macro) and transient response that would see most typical hifi-speakers crumble into smoke trying to achieve. Hearing cleanly reproduced dynamics and transients approaching live levels might deceive some into thinking it sounds "harsh," simply for the reason that they've never before heard it via any typical hifi-speaker, which are prone to seriously "mute" and lessen the impact of dynamics.

But remember, it's not so much about max. SPL's per se as that feeling of effortlessness, low distortion, ignition, and the sheer area of displacement afforded by the bigger units and horns/waveguides. Heavy metal needs to be felt as well as heard.

I don't agree that speakers that excel at full-scale orchestral will also excel with metal. Especially current metal, highly compressed and recorded digitally. Even during sustained crescendos, metal is full of textures and sharp transients. Not many speaker/amplifier combinations can deliver sustained high sound pressure and also maintain those transients.

I get that you don't agree, but you would be wrong, except for that last bit, which I agree 100% because that just happens to be true of most audio systems.

Orchestral material has plenty of sharp transients. Tell you what- find your self a copy of Das Reingold, on Decca or London ('blue back'), conducted by Sir George Solti. Put on side 6 about halfway through and crank it up as hard as your system will take. Make sure you let it play all the way to the end.

When Donner's hammer creates the Rainbow Bridge- its a bit of a transient :) Its also pretty metal. Norse gods- what can be more metal than that? But hang on till the final fanfare! Many systems just can't play this disk at full whack. You need power in the amps and efficiency in the speakers. That's just the way it is, regardless (for the most part) of the music genre. If you want electronia to sound its best with all the bass impact, you need exactly the same thing. I can't get my own band's records to sound right unless I crank them up too. Some music just needs power, without coloration. Metal is nothing special in that regard.

"That's just the way it is, regardless (for the most part) of the music genre. If you want electronia to sound its best with all the bass impact, you need exactly the same thing."

That is true.

Of course its always hard to define what is good and what isn't. In this case its better to talk about performance in regards to ability to deliver the most challenging musical passages at high volume. You will probably almost pay more for better performance on a larger scale, but what sounds "good" or not is subjective and way more up in the air.

If you are reproducing music only on a small scale and with limited volume, then the world is your oyster. :-)