You don't need lab tests to confirm that, it is a simple physics. No localisation occurs because woofer diameter is much smaller than wavelength of 80Hz wave and below. It happnes gradually, of course. If membrane playing 80Hz and below would be large enough those waves would become very localised.
Anyway, that means that answer to your questions is that multiple sources for low bass doesn't spoil anything related to "virtual sound object" nor localisation because their woofers are far too small to be localised at frequences below 80Hz where they play.