If you can 'locate' the single sub easily, then of course it affects the imaging. Even if it does not technically 'pull the image' to one side, the ears ability to locate the bass affects the image in your brain. If properly set up, a single sub can often still easily be located. Just close your eyes and listen. If you can point right to the sub, it it not nearly as ideal as two subs. In both of my listening spaces, my pairs of subs are located well off the the sides and are not even easily seen from my listening positions. Placing them here made a huge difference compared to right beside my speakers, but it was for decreasing booming not imaging issues. Congrats on turning the lowpass filter down to help!
Subwoofer Question
Could having just one subwoofer (REL T/7x)with the crossover set around 60hz pull my image to the side that the sub is on?