The big oversized ENGL cabs will get the job done, I'm sure. Well made and dependable. If the ENGL dudes let you in the front door, try and sneak some photos for us! I'd love to see their HQ.
The HiWatt cabs are a good suggestion. I've seen some of the Doom/Stoner guys using HiWatt amps and cabs to great effect. Sunn amps and cabs as well. The original Doom Metal sound, as far as my (limited) knowledge of the genre allows. I think a 15 inch speaker will do you well. Randall used to make a 1x15 guitar cab, the Warhead I think? or X2. You could also grab a bass cab too. Nothing wrong with that. I really like the sound mixing a 2x12 with a 1x15 for any sort of music really. It really opens things up, IMO.
You could give Avatar Speakers (avatarspeakers.com) a call and see what they can do for you in terms of a 15 inch cab, but the shipping from the US to Austria will be brutal on the wallet. I'm a big fan of their cabs, and they are quite cheap. I've recommended Avatar cabs to everyone who's ever asked on this forum I think, haha.
The OS PPC cabs by Orange are worth looking into as well. The Avatar Contemporary cabs are very similar to the Orange stuff both in look and sound, for less cash. When I think of Stoner/Doom, I think Orange/Matamp/Sunn equipment.
What amp(s) are you running through? Some more info on your rig will help narrow down what kind of cab and speakers will work best.
Good luck on your search for the ultimate in Austrian Doom
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Edit - If you don't think 15 inch speakers are good for anything other than bass or teh br00tlz metal, let it be known that Joe Perry uses quite a few 15 loaded cabs in his crazy live rig. I'm not a big Joe Perry fan, but you can't deny his importance. His rig is crazy! Check this stuff out. 20 feet of amps and cabs, all being used on stage. Oh, how I wish I was a millionaire.
https://www.youtube.com/watch?v=5P8EqX3 ... 1824494597
