What does the room sound like? What are you trying to reduce? Bass, mids, HF, reflections? The treatment you buy or build does a specific job. 25mm thick foam might reduce the very top end from maybe the percussion shakers, somebodies excess sibbilance from a gap between their teeth, or over bright tweeters in a home hifi speaker. Those speakers are somewhat odd. electrostatic speakers can be very good but I've rarely seen a website where they don't put response curves in a document somewhere - these are full of flowery descriptive words that mean bugger all - so I have no idea what they sound like. Most speakers used in home or smaller speakers display proudly their frequency response so we know what to expect and where to watch for issues? These are visually very nice.
Your concrete walls - are they smooth our rough - one reflects badly one far less so.
I can only assume it's for a hifi listening room - where your taste will be more important, but foam and other absorbers get VERY thick when they are soaking up bass, and sometimes foam becomes unmanageable, so bass traps might feature membrane absorbers but they tend to be very frequency specific in terms of where they are good, and where they are pointless. They often end up in the corners where the square shape rooms concetrate energy.
In short, you need to play some music, work out what needs treating and then buy exactly what you need. It may possibly be sorted with a large area of foam - 50mm/2" would be more common, 25mm is no good for medium frequencies - they pass sraight through. You might end up hanging big rockwool stuffed panels on the walls for mid frequecy control and then bass traps - but the first question is always what does it sound like? What do you want to control, and then you can plan and buy. For hif fi folk with their slightly different hearing to ours, visual treatment often makes it sound better, especially if they have spent lots of money, but sometimes money gets wasted because nobody identified what the problem was. Download some pink noise on youtube and use a phone app to tell you what you are hearing. Look for odd humps and velleys in the response. You should get a straight line, but you won't. Your speakers, for example appear to start working at 100Hz. The Sony typical sub has lost it's energy by 70Hz or so - so you might hear little in that gap?
