There's also this piece of (what seems to me) great advice from my sound-technician friend in Sheffield. I posted it on facebook, but here it is for anyone who doesn't go there:
It's good that you're having tinny, harsh acoustic problems, because they're the easiest to solve! Basically, it's caused by surfaces reflecting the harsher high fr
equencies, which are much easier to treat than unruly low frequencies.
Everything you've suggested makes perfect sense. Basically anything soft will dampen that harshness, and the more of it the better. The tiles and windows are likely to be more troublesome than the concrete, so if you're at all worried about aesthetics, leave the ceiling until you've tried treating the other surfaces.
One thing to bear in mind is that parallel surfaces are more troublesome than non-parallel surfaces. For example, if the windows you described are on opposite walls, the reflections will have chance to bounce back and forth several times before being dampened. Also if the ceiling is horizontal, that will have the same effect in conjunction with the floor.
Wood is better for absorption than tiles, but any kind of carpeting is better still - even thin carpet tiles. Better still, a thick rug on a carpeted floor!
And yes, for the windows, heavy drapes or curtains would be best. The more of the glass you can cover, the better.
For the ceiling, there are a few options if it's necessary to treat it. You can get a company to carpet it (quite a few night clubs have their walls carpeted for a more immediate sound). Alternatively, you can attach rockwool or anything soft - old duvets etc. - to the ceiling and cover it over with fabric. We did this in our studio, and it worked wonders. Acoustic tiles work brilliantly, but they look a bit industrial. Egg boxes are not the best solution, but they're free, easy to fit and will make some improvement on the concrete.
All these solutions will also shorten the reverb time of the room. If the reverb tail is nice at the moment, you might not want to over-do the acoustic treatment. But if the decay isn't worth keeping, then do as much as it takes to temper the tone of the room.
Every room is unique, so whoever takes the lead with the work will learn a lot about the room and about the effect of the materials he or she uses in their experiments during the festival. I'm by no means an expert, but you're most welcome to share my email address with anyone who might find it useful.