The big problem with isoboxes, as I've learned to my pain and detriment, is that no matter how good the isolation of the box proper might be: you still have to cool it. And if the cooling airflow is noisy, what was the point? I just this very evening put a Raxxess Isoraxx 14-space hushbox on the truck to return to the vendor. My finding was that the box was *noisier* than the equipment I was putting into it!
Now, there are some spectacularly noisy computers out there, to be sure, and those would indeed be helped by that box. But the big problem with the Raxxess box was that the air handling system inside it made just as much noise as a production computer case. No win- in fact, as lose, compared to a modified computer with properly-tuned cooling.
My conclusion is as follows: you have to move air to dissipate heat and keep modern high-tech equipment alive. This can be attacked by moving the air more slowly (which implies a larger aperture for the air to move through), or by moving it into a space that you aren't in (sticking the machinery in another room, or venting the "hushbox" into a separate space from the area occupied by your mics). Or, as I've decided to employ, you can move less air more efficiently, and decouple mechanical noise sources from the environment in other mechanical ways.
Now that I've had a chance to work with this a bit, I find myslef tending to reduce fan speeds to the minimum that can support survivable cooling, and to add mass and damping to enclosure materials. The sheetmetal cases for most PCs, and more than a few professional studio pieces, are often large unsupported sheet metal spans with no damping... These will ring like *bells*. Think of the exterior sides as 19" by 19" speaker cones with your disk drive as thge voice coil! A small amount of damping can keep acoustical energy from crossing the barrier presented by the case (by killing resonances), and adding mass can alter the resonant frequencies: often enough to get a problem piece of gear to hide in the HVAC noise of the room.
Turbulent airflow noise, the "whoosh" of most PCs, can be dealt with by reducing the airflow to the minimum necessary to keep it alive (and adding internal baffling to focus that airflow to where it does the most good, over the heatsinks and SIMMs). Mechanical vibrations, the pitched "whine", can most effectively be dealt with by decoupling (mounting the source on compliant isolators, like rubber pucks to mount a disk drive carrier) and by adding mass and damping to detune and reduce the Q of any resonances.
Some whines are airflow-related, like the small-diameter CPU cooler fans that often operate in pretty much stalled air (and whine as a result of their blades encountering their own turbulence). For those, the best that can be done is to reduce the fan speed to the point that the blades aren't stalled. They are probably more efficient as coolers in that case as well...
The more you pay for a hushbox, the better designed the cooling mechanism will likely be, and the better your results. However, ultimately you're putting a box with fans inside a box with fans. I was disappointed with my low-end hushbox experience, but in retrospect I should have known better! There ain't no such thing as a free lunch.
