I'd experiment by pulling the chair away from the small corner. Don't be afraid of nearer field listening to further reduce room effects. Then apply treatments, but only as needed, even Toole listens in a quite ordinary room at home. The western ear is acclimated to listening in rooms, again of the ideal shape (in ratios of 5:8, height to width and width to length). Treatments can be in various forms but generally fall into two camps: absorption and diffusion. Absorption can be a simple as stuffed furniture and can be overdone (sounding muffled). Diffusion is simply based on laws of physics and the size of sound waves, so most commercially available products are only effective down to roughly 1100 Hz (7 inches deep), and many aren't very effective as they aren't made of reflective materials. I use 3 tall randomly filled bookshelves on the side walls half way back into the room.
Finally DSP (or EQ) should only be thought of as the icing on the cake and then only carefully applied. These can only be effective at a single location in the room. They absolutely shouldn't be used to make up for poor speaker performance. And if done by ear can really mess up the sound and even destroy your gear, because deep dips cannot be addressed. Deep dips in frequency response, like -30 dB, which can occur requires 1,000 times the wattage to compensate due to the logarithmic relationship between power and sound pressure. This 1,000 fold power increase would clip the amp at higher sound pressure levels and send a torturous signal to the speaker, likely frying the voice coils out. If using DSP/EQ, best to do it with unit that includes a microphone and does it automatically. Note that a narrow dip can't be heard anyway.