Hi o,
If moving your tubes moved the image, then they are effecting much more than low bass frequencies since these are non-directional and the bass nodes are larger than the width of your room.
Diffusion is primarily to improve imaging by scattering and diffusing wall reflections such as to dissipate them. Absorption is actually removing sound pressure and signal level at certain frequencies. So, if your room is tonally correct as set up, then adding diffusion on the wall will reduce early reflections and improve image specificity (remove the boundaries). If not, then absorption (applied correctly) will improve the spectral balance of your room.
Every audio system inside a normal size room has numerous reflection points along the walls and the ones that occur at the first reflection point, midway along the wall between your speakers’ baffles and your listening position, are the most detrimental to imaging specificity, so something should be done to reduce/eliminate them. Ignoring bass for a moment, if the sound is too bright, then absorption at the first reflection point (midpoint along the wall between the speaker baffle and your listening position) will help to correct that. If you feel that it is just right, then avoid absorption and go with diffusion at the first reflection point, lest you suck the life out of your sound.
My thinking, right or otherwise, is to address spectral balance first (absorption). I suggest you begin determining what your room is doing acoustically regarding the spectral (frequency) range. Do you have a test CD with different test tone frequencies and a RatShack analog sound pressure meter?
Best to you o,
Dave
