I recommend UMIK-1 USB microphone (about 100USD) with REW free software, cheap and that can measure everything you need, volume control, level meter built into the REW application. DEQ2496 has analogue microphone input and it is possible to switch it to the output, but I don't see reason to work on this way.
If room and speaker equalisation is your target, Behringer has RTA - real time analysis, room or speaker frequency response measurement capability built in, which works very well, but that is good for room amplitude equalisation only, no phase, no waterfall, step response, THD curve measurement capability, if your intention to measure these on your speaker. So I recommend Behringer against room modes with internal RTA, but UMIK-1+REW to measure speaker parameters.