@ Gavin, I would suggest addressing the first order reflections next. This will go a long way in improving spatial imaging and tonality especially. If needed, use absorption at these points to also address reverberation control, otherwise use diffusion or a combination. The ideal would be an attenuation of about 15 dB at these points, and as broadband as possible. Try to keep the left and right horizontal plane symmetrical. Acoustic treatments could be as simple as large leafed plants, staggered books, etc., or true acoustical treatment products.
Current room is 16 x 24, but hoping to do some remodeling next year and increase it to 24 x 36. All of the most realistic systems I've heard have had one thing in common, a fairly good sized room. I just spent time with our writer Jacob Heilbrunn (who also writes for TAS) and he's got a pair of Wilson X-2's in a room that's about 20 x 35 and the presentation size was fantastic. Dave Wilson also has about a 30 x 40 room that is very good as well. A few others come to mind, but the one thing in common to really get that last bit of lifelike perspective is space. The final frontier.
Of course this isn't terribly practical for most people!