My "sub" is a cobbled up thing - it's a Kef Concerto enclosure (BR with about 50 litres (1.8 cu ft)) with two B139 drivers back-to-back in isobaric instead of the usual one driver. I needed to check the tuning - glad I did as the port extension I had added was way to long as it turned out. The -3 dB point is about 28 Hz, which is not a big advantage over my main speakers actually.What are the details on your sub?
The response above is for the amp, measured into a resistor.
I wasn't game to use REW for this as I didn't want to connect the SD 300 output into my brand new sound card, since the speaker voltage is sitting at half mains supply - so 120 volts AC above earth here in Aus.