Id not spend too much on it until you know what you want to to sound like. Much of the sound from an amp that you hear is reflected sound and you may find that the ambiance in the iso-booth doesn't work for you.

I have a friend who went and deadened all the reflections in the room he records in and ended up ripping it all out as it sounded horrible !!

Have you checked out the aurolex web site ? The have a lot of acoustic info there that might be useful. other useful places to check in on would be www.homerecording.com or www.recording.org