hi OV,
Thanks for your patience -- we've gotten a ton of inquiries over the holiday and we're getting back to everyone as quickly as possible.
I'm always a bit suspicious of "acoustic curtains". Not that they don't help, but most fabrics will absorb at least some treble. The question is how much absorption, and to how low a frequency. High frequency absorption is the one thing we can overdo when treating a room.
Strategies will depend on where the archway is, relative to the speakers and listening position. If it's not in a critical area then I would not prioritize it for treatments, and just use whichever curtain you like the looks of.
If it is in a critical area like a reflection point, then we'll probably want to use some freestanding device that is more effective than a curtain.
Once you hear back from the GIK designer, you can follow up with them for more specifics depending on your room. I'm sure we'll get back to you soon!
JWL