I have a 2x12 Diezel RL closed-back cab with Celestions and the new Diezel Hempcone loaded 2x12 open back cab.
I use them with an Einstein combo and a Schmidt.
While the Celestions have a really good tone, I have come to love the Hempcone ceramics much more. They don't have as sharp a treble edge when the treble is cranked, but have a sweet, natural treble tone and they sound more natural and open to me. I play in a 5 piece rock band and either amp sounds perfect, but I have come to love the Schmidt the best, due in part to the speakers and the sweet tones I get from the Schmidt. They just sound more natural and refined to my ears after trying both speaker cabs.
I might be an acquired taste, but for me it was using both types of speakers and listening to what I got from each type of speaker. As each of us hear things differently, it will depend on the styles, volume, and guitars you try them with, which will lead you to your own conclusions and preferences.
I don't play metal, so I cannot comment on how the hemp speakers react in that situation. However, I totally trust what Peter tells me, and have always loved the amps and speakers he reccomends.