How profound a difference is a .5m speaker move likely to make to measurements?

Other than moving my speakers .5m closer to the back wall nothing's changed in my room iro acoustic treatments or their location and it's been ages since I measured.  For shits and giggles I figured it was time to experiment with using REW to create convolution filters, so out came my Umik-1 and tripod.  First thing I see after opening my mic case after getting it back after it was out on loan it is that the serial number's been torn off.  Hmmm, fck I mutter, and set about finding the calibration file on my PC.  In luck, I find it and apply it.

I then ran a sweep 0 - 200Hz sweep and reviewed the graph. :OMG:, the bottom end is out of control and nothing like earlier graphs showed, nor does it correlate with what I hear.  So I'm left wondering whether this is in fact my mike, and if it is, has it been dropped and gone to pot, or could a .5m move of the speakers cause a fundamental change in LF response...

Thoughts from those in the know where room acoustics are concerned?