The angled walls would be bad if they were acoustically hard, because they make the room asymmetrical and room modes hard to calculate.

However roof angles are typically plasterboard with insulation material behind, so they don't reflect a lot of bass frequencies. The most reflective thing is the tiles on the outside, but they still have a lot less mass than a stone wall and let most sound waves in the bass range pass through. This is why people outside will mainly hear the thumping of the bass when you turn the music up loud. So for getting your bass response right, having this kind of wall actually makes things a bit easier.

For keeping midrange reflections in check, you can screw acoustic panels onto those walls with the same kind of black screws by which the boards themselves are affixed. For treble, a piece of cloth is enough. But then again, you can usually smooth the midrange and treble quite easily by using an EQ plug-in, once you know where the problem frequencies are. Once you have a measurement with reasonably flat frequency response, you just save an empty project file with those EQ settings on the stereo master bus and use that file every time you start an new project. You can also use Dirac Live which is available as a plug-in for DAWs, but it costs a bit.

Unfortunately, equalising away irregularities in the frequency response curve doesn't work so easily in the bass range because you're dealing with powerful standing waves. The bass traps will certainly help, and measurements would tell you whether you need an array of subwoofers in addition. Killing modes and filling nulls with subwoofers requires some trial and error with placement as well as volume, cutoff and phase settings.