Yes , i took the FR of the Mains alone first then compared to 2 sweeps of the Subs ,one at 12' and the other at 18' , they were identical compared to Mains ??Delay settings will not make any difference to the frequency response of the sub alone, it is only in combination with a main speaker that you will see an effect in the response.
I am using a USB microphone and my laptop sound card output. Will loop back be effective in this setup as there must be a DAC delay in the mic different from the sound card? Is the RTA analysis you mentioned the best work around?You won't see a difference in frequency response unless you measure with both the sub and a main speaker running, the delay changes affect how the sub response and main response add when both are producing sound. If you are looking for timing shifts in the impulse responses you need to provide REW with a timing reference by connecting a loopback between the unused soundcard input and output channels (e.g. the left channels if you are using the right channels for measurement) and ticking the box on the Analysis preferences to "Use loopback as timing reference".