I wrote this some time back and never posted it but because you asked I’ll post it now.
Now that I have heard both the F15 and a FV15, well not really a true F15, I have a DS1505 that I switched from sealed to ported. My sealed enclosure was a little larger than the F15 which I’m told makes it play a little lower, but the ported is the exact dimensions of the FV15. The reason I did this is I never thought the sealed blended well with my other speakers. I have all Klipsch Synergy speakers with the mains being the big ported F3’s. They have been described as harsh and in your face and to a certain degree this is true. I would describe the sealed sub as being very smooth and mellow that doesn’t mean that it won’t get loud, I could turn up the gain and over power the F3’s that have a sensitivity rating of 97db, but it just didn’t blend right with them. So I talked to Brian and he told me how to convert the sealed to ported, as most of you know he was very helpful. I would describe the ported as more powerful and fuller and it matches and blends much better with my F3’s. So when you are deciding on sealed vs ported and 12 inch vs 15 inch your room side does matter but more important than room size is what speakers are you mating the sub to and how they will blend with each other. It’s not all about output the sound signature of the sub must match the rest of your system for it to sound its best. As you can see from the graph that as for as output goes there’s really not a night and day difference, by about 37Hz up they are the same. But there is a difference in the way they sound so you can’t tell everything from the numbers alone. Sorry but I don’t know what the extension filters were set on for these measurements they were taken months apart but from the same listening position, if I had to quess I would say the sealed was 20hz mid and the ported 14hz low but that is a quess.