10dB resonant peak at 40Hz

OP:

Yes, that’s the one.  As you mention, plugging the ports will work but sometimes at the expense of all lower bass.  A parametric EQ on the other hand can clip these enormous resonances effectively and make it possible to better balance the bass output.  With subs clipping peaks can often help you raise the sub volume very effectively.

Well, if you could move the speakers I’d say use the AM Acoustics room mode simulator to try to put your speakers and listening room out of the 40 Hz mode. Otherwise EQ is your only option.  If it makes you feel any better, it's a very good option as well.