You could use REW which will accept two different devices for input and output.
But yes, in theory the Dragonfly is sufficient for the task along with a USB soundcard.
But then, any USB soundcard is also sufficient as it also has a low impedance headphone output which will not be much worse than the AQ.
In theory, at least. In practice, bus powered USB DACs will not be really able to provide reasonable output into a 8 ohm speaker load as they are designed for minimum 16 ohm load. Also the usual output cap is 100uF, which will have a profound effect on the LF measurement.
For tweeters there will be no issues.