Glass can be hideous for hf resonances in a room and I remember our dem room years ago (KJ W1 in original Wigmore St site) had a massive issue at 5khz with the 1960's glass panels resonating, so we put café-style curtains up and replacing the vertical blinds there before to minimise it - all gone now as the façade to the street has been restored to original form and the entire building behind it razed and rebuilt as is common in those parts I gather... Apparently, the now heavily 'windowed' BBC radio studios at BH do this too, hence choice of close directional mics for the 'presenters.'

Seems to me that very heavy curtains could easily stifle the high frequencies - maybe that's a benefit to you ?
In my room, large medium weight curtains do affect the treble, but equally when drawn as when open, so you may not have to actually close the curtains to get some effect..

The thing is, so much current fashion seems to call for bare walls and floors, it can play havoc with sound systems. Killing a room stone-dead isn't a good idea either unless you'd be monitoring or mastering tracks in it, and even then (the not large mastering rooms at Decca were great for 'work' but sterile otherwise and rather airless in every way... Interstingly, dealer dem rooms could be the worst of all I remember - and I visited many in my life for work and pleasure...

One advantage with taking gear to bake-offs is that you get a good idea of how known gear sounds like in other totally different rooms, and this may help in furnishing and 'setting up/optimising' your own. The more costly one's stereo, the more important the room becomes otherwise why pay so much hard earned dosh on it?