Well i've already came across some possible solutions
1) Installing 2 absorption panels on the front wall and:
- Surrounding the rest of the studio with sunburst 360's. But i'm not sure about the bass here.
Sunburst 360s - Acoustic foam sound absorption products from Auralex Acoustics OR
- Using moving blankets as side walls, just like my curtainsolution, but i don't know what to do with the back wall here since there are closets there. Put panels on the closets?
2) ARC: I've read good things about the ARC system and i've read bad things about the ARC system. But apparently it's a huge improvement for an untreated room! And since i'm not seeking for a high end studio at all, this would really save me out of my misery! I'm going to do some more research about this, i can always combine it with a little treatment. And this won't ruin my decoration

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I still have to do a lot of research about treatment, so if these are bad solutions, i would like to know.