It is difficult to get accurate distance measurements from subwoofers, an accurate measurement needs a wideband signal, which a sub cannot reproduce. If the subs are the same then easiest check is with a tape measure to the listening position, or play the Pink PN test signal and play with delay settings while looking at the RTA plot, set for Rectangular window and no averaging.
Maybe I should have worded it differently. Can I use rew to check the time delay between one signal and another I.e measure one sub and then measure the other and see if there is a time difference. Regards lee
Possibly, though different locations can produce impulse responses that are sufficiently different that they are difficult to directly compare. Try setting up a loopback connection on the soundcard channel you are not using for measurement and tick the box in the Analysis preferences to use the loopback as a timing reference, then compare the impulse responses on the Impulse graph of the overlays window.