It depends on what exactly you're trying to achieve here. It seems to me you want to keep some liveliness to the room, and that's probably a good thing. Dead rooms sound unnatural. Insulation is an extremely good sound deadener. If I am correct in my thinking that's not what you want.
If cost is the reason you're wanting to go with insulation, you are looking at the wrong vendors. Auralex is extremely overpriced. I've had very good results in the past with foam factory. With 1“ pyramid panels you could treat even the first reflection points and still keep some reasonable amount of first reflections,whilst taming those third, fourth, and so on reflections that cause the fluttering. The important part would be making sure that your brain perceives the sound coming from the speakers as the originating source. Part of this relies on timing, but even more of this relies on volume. In small rooms, the timing differences between the sound coming from the speakers and the sound coming from reflections is reduced, since there is less time for the reflected signal to decay there is less volume difference as well. Utilizing the aforementioned panels will simply reduce this but still allow you to experience natural sounding reflections. Thin insulation is likely to deaden anything at or above 1/4 wavelength, effectively killing off most midrange and highs. The pyramidal shape and porous nature of acoustic foam still allows some reflection, though at a lower volume level, and would be more appropriate for what you're trying to do. Placing them parallel to each other in the room would effectively kill the flutter echo while still leaving much of those initial reflections intact. If it were me and my room was untreated or treated only with bass traps, I would go for 8 24x48 panels, two behind the speakers at the front wall, centered at ear height, two parallel to each other on the side walls at ear height, equidistant to each other, preferably at the first reflection points, and two on the back wall, parallel and equidistant. You might also consider 4 12x12 panels evenly space on the ceiling.
You said the back wall is treated already? What else is treated and where? What is it treated with?