#1
Due to having neighbors, I'm thinking of building an isolation box for my 2x12 cab. I play guitar, mostly death metal (think Nile, Cannibal Corpse, etc). If you play death metal / bass and have used an iso box could you tell me how much bass you loose with a fairly large box? (I know the little prebuilt ones with a 1x12 don't work for metal.) Also what is the least amount of space I could leave in front of the of the cab without loosing much sound quality?

Details -
The cab. is closed back so only the space in front will make much of a difference.

I'm planning on lining the inside of the box with acoustic foam, rounding off the front corners, and making sure the box isn't square for good sound quality.

Yes, I really do need an iso box to record and even play on my amp. My walls are so thin I can hear my neighbors talking, watching TV, etc. and for some reason they don't like death metal at 3am.

Yes, I already have a hotplate, it only keeps my eardrums from bursting not the cops from showing up. No, I don't want a solid state amp, my tube amp kicks a**.
#2
I wouldn't say that a 1x12 isocab cannot be used for metal, Ola Englund gets some sick tones using a Randall Isocab.

http://www.youtube.com/watch?v=osCOjDwbhyM

The main thing with an isobox is less the loss of bass frequencies and more of an issue of having to add reverb during mixing due to there being little natural reflections.

Also, even with an isobox you still won't be absolutely quiet as some sound will still bleed out, but it will help keep down the overall level of noise.
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