Plug the port. That's the easiest thing to do.
Your other option is to lengthen the ports. How long? 60" That's right each port would need to be 5 feet long to get your tuning down to 25hz.
I'd sooner make a new box.
Today I plugged the ports with the foam and gave it a test run. I'm quite sure the frequency response is better than it was. I've reconnected my ep2500 and giving it much more power now.
I'm very confused about what I'm supposed to hear between 20-25hz. I get lots of structural vibration in the house from 20-24hz. Practically everything that isn't tied down is rattling and it's not until about 25hz that the tone is much more audible. 26,27 begins to get exponentially louder and 28hz is like a sweet spot. Are room accoustics / low room gain playing a part here ? Are subs with lower Fs and higher xmax parameters very audiable at these frequencies or do they just shake the house panels apart like mine does and the tone is only barely audiable ?
My previous living room had hard wood floors and low ceilings. I recall playing a few favourite tracks with low 20hz tones that even my 10" mission m6as sub could pull off. Now that same song, same sub in a different room and it's completely missing from the song.
I also have some serious peaks and nulls in my living room. I'd be interested to graph the frequency response of the sub at my seating position. Is there a cost effective way to do this ? I don't own an expensive calibrated microphone.