I don't think it is microphonics. If the speakers are placed within 8 inches of the wall the bump is gone and so is the rest of the soundstage.
I may have misinterpreted this statement and the fact that you system photo seems to show the tubes quite close to the speakers and close to a corner (more bass energy there) and on the top of a stand (potentially more vibration).
I have not been able to find a room placement that is livable that will reduce the peak below this point.
Your description is sounding more like a classic case of quarter wave cancellation off the rear wall.
It is almost impossible to beat with absorption as your speakers and the listening position are generally symmetrically positioned with repect to this very wall. You get very broad and coherent dips and troughs right across your room. (This applies to every sound that has a quarter wavelength corresponding to the speaker distance to the wall...AND EVERY MULTIPLE of these up to around 500 or 600 Hz at which point sounds are more directional and tend to only go forwards from your speakers)
Now you know why Studios place monitors in walls.
If you don't like the sound with the speakers right up against the rear wall then move them at least 8 feet out from the wall and the problem while still there will be diminsished.
I hope this helps ;-)
Good luck.