I am surprised, would love to see this written up with actual measurements of the difference. I'm pretty sure we could see what's going on in the frequency response.
Subwoofer modification yielded excellent results.
I have dual sealed Dayton subwoofers. I began to wonder what would happen if I drilled holes to relieve the pressure and allow the speakers to achieve more excursion. I don't like the ported 'one note bloat' sound is why I didn't go for a ported design. I see now that a passive radiator is a better option since it alleviates the internal pressure allowing for more cone movement but I have what I have.
So I decided to drill a small 3/8" hole in each corner of the face of the Sub box. I did the front for easy adjustment but could be on the back side so you don't see the holes. (total of 4 in each box) Using rubber corks to plug them up to test and tune the box pressure. No big deal if it didn't work so it seemed like a worthy experiment.
The result was amazingly better bass. No sound is emitted through the small holes but you can feel the air moving. This does allow for more excursion. The bass is tight and MUCH improved and can be tuned using the corks. The holes are unnoticeable.
The combination of proper phase, connecting to the 'high level inputs' as Paul McCowan (and Rel ) suggests and drilling holes to vent have increased the quality of the bass output enormously. Also required LESS power since the speaker can now breath better and produce more bass. I have no regrets and am VERY pleased with the results.
Again, if there is no change in sound the sub box can be restored simply by plugging all the holes with the rubber corks and you're back to original condition.