There are several systematic methods of tuning by listening that work, more or less, to get the set up close enough for purposes of experimenting with optimal placement. The Sumiko method optimizes placement for best bass reproduction and balance. The other useful method is the Wilson method, which places emphasis on midrange clarity and naturalness. Regardless of the method employed, there will be room for tweaking. You mentioned experimenting with toe-in, that is good because proper toe-in is crucial. You should also experiment with rake angle (how much the speaker tilts back).
Room treatments can be useful, and if one cannot do this for aesthetic reasons, the treatments can be nothing more than a few tapestries or other decorative items on the wall. Book cases and other objects along the wall also tend to add reflections that improve the sound. Big expanses of empty walls is usually not ideal. I would certainly try something on the wall behind the listening position. Carpets or rugs are very helpful for breaking up the back and forth reflection between the ceiling and the floor. If you don't have a carpet, put a rug down between the speakers and your listening position. The table between your speakers and your listening position can also be a source of undesirable reflections; you can check that out by temporarily moving it. If it is a problem, but must stay, you can ameliorate problems it causes by putting things on the table to break up the reflection (books, etc.).
Most of the rooms I've helped with setups were improved by having large plants in them, either real or artificial. At big shows, like CES, many exhibitors buy large potted plants to put into their rooms (to help the sound, more than for improving the looks); after the show, they trash them.