Thanks mate. Yes- the breathability of the room is also worrying me. With a group of people all playing away, humidity will be an issue.
I'm going to ask as many professionals as I can. The few I've spoken to thus far have either tried to sell their product, or seen it as a challenge, but like you say, if the challenge defeats them, I'm left with an expensive and unuseable space.I appreciate your replying - muchos danke!
The word professional is banded about the place with very wide meanings but in this instance I suggest you consult and pay for professional whose product is their advise (hence the iffs buts and maybes) if the task goes beyond you risk zone (should be well beyond the comfort zone: )
Breathability is one of those buzz words that have left construction academic papers and colonised sales depts.
In the case of your sound room a simple extract fan might be much more effiecient, practical and cheaper though less eco-exotic. Many of the current condensation problems are a downside of technological improvements eg plastics (inc. plastic paints)and double glazing etc.