The primary goal of my subwoofer purchase last fall was to provide as even of frequency response as I could, as low as I could, in my main shared space where our "home theater" resides. This is a fairly large space in and of itself because it includes the kitchen and a large fireplace nook, and it opens onto another shared space that contains a dining room and main living room of the house. There's no way to seal it all off.
At the same time, I'm not the sort of person that listens to things regularly up over 100db. There's loud, and then there's dangerous. I've done my time in various lay PA and theater environments (usually with earplugs). I don't want that at home.
In a nutshell this all means that I want things to go deep, and at a reasonable sound level.
My mains afford me the luxury of going down to 50hz without issue. In fact, on most material other than serious movie offerings or some of my electronic music, the sub can be turned off (though I rarely do).
So with this in mind, I run my VTF-3 MK4 at EQ1 (to benefit low frequency extension), one port open (lowest tuning frequency, for more low frequency extension). Initially I ran with a Q of .3, and by all means loved it, but I ran into some serious room gain between 50ish hz down to 30hz hz. I didn't mind so much. It wasn't flat, but meh, neither are my ears.
Months later, a bit more experimentation. I bumped the Q to .7 based on the nifty graphs on the VTF-15H page (might I suggest generating the same type of response curves for the VTF-3 MK4, Dr?). This caused an even more obnoxious gan in some of the lower frequencies, but what it really really did is bring out the suuuuuper low stuff (down below 25hz).
So I lowered the gain from it's usual 9:30 location back down to about 8:45 and recalibrated MCACC on my receiver. I also told the receiver that it could bump the crossover point up to 80hz if it wanted. I was curious as to how it would equalize everything out.
Once done, with the receiver no longer considering the mains full range, and part of the MCACC equalization going towards the subwoofer now... things changed. That huuuuuge hump I used to have was now mostly gone, and the presence of the super low frequencies stayed. I bumped the gain on the sub back up to ~9:00 and have left it ever since.
The more you ask equalization to do for you pre-amplification, the less the amp is going to have in reserves when things really get crazy. Port gain doesn't have this problem because it's purely acoustical manipulation of the waveform after it leaves the cone. Again though, EQ1 vs EQ2 and any of the boosting that running the Q above .3 gives you is a direct drain on amplifier capacity.
However... this really only matters to you if you run your sub aggressively.
I've done some tests with this setup with the gain ran as high as 12:00-1:00, and frankly it's scary. Not because the sub can't do it. Not at all. The sub is just jaw-dropping in what it can do up there. But doing this puts things up at 100db or better in some of the passbands, and frankly when I can feel the drywall flexing, when my chinese porcelain tea set is trying to walk itself off a shelf, when pictures are trying to hop off of wall anchors... it's too loud.
Rooms vary, and if you have a smaller space, the room's going to help you acoustically reinforce lower frequencies. All things being equal, you'll only require less aggressive Q settings to get solid frequency response down in the crazy of the lows. As the room size gets bigger... you may need more help, and that's where you'll want to transition to EQ1, one port... and start bumping Q.