[JohnM]

From your descriptions so far you have been evaluating alternative sub positions by ear, but if you do not adjust the sub level relative to the mains for each new position you are wasting your time. The graphs you have posted of your sub relative to the mains show it much too high in level compared to the main speakers. It will draw attention to itself because of that, leading you to reject positions because the levels are wrong rather than the performance is not right. The measurement in your first post shows a small resonance, adding about 4dB at around 48Hz, and several others above the crossover where they are not having much effect on the overall response. The bulk of the sub response in that plot is simply the natural performance of the sub in the position you have placed it when measured where you put your mic, it is NOT a room peak and it is high because the volume of the sub is turned up too much.

To properly evaluate an alternative position take REW measurements first, to see if the position is helping to increase the output below 25Hz where it currently rolls off. The only way to raise that will be to get the sub closer to a wall or into a corner. If the output at low frequencies looks better in a position go through the normal sub/mains level balancing first then listen to how it sounds. Getting closer to walls or into corners will probably result in some peaks in the response as room modes are driven into resonance. A BFD could help reduce those, if they are below 80Hz or so you would need enormous bass traps (several feet deep) to achieve a similar effect.