You haven't mentioned your SPL meter, or what sort of test tone you are using but those port plugs and subsonic filter are very much interactive (there are EQ tweaks engaged at the same time but we like to keep the discussion simple ;^). Just don't mismatch settings and you should be fine.
One plug... with a 16Hz SS filter setting isn't that big a deal. But you should not "compensate" by turning up the sub, rather, calibrate with that meter as you normally would. Heck, you might even find the 12Hz setting is to your liking. Just don't run the 12Hz setting with no plugs! (rather the requisite two ;^)
It's true you do lose some headroom CAPACITY above the tuning point with this sort of configuration but the PC+ is pretty capable, and you might not ever discover the limits when things are properly configured. IF you have a meter to keep things honest (do not trust your ears in other words) calibrate with the 20Hz mode and do some demos. Then, changing nothing else, try the 16Hz setting, calibrate, and do some more demos.
Eventually try the 12Hz config but keep in mind this is very much a worst case setting on the sub and will indeed reduce headroom in the common 25ZHz and up region a good bit.
Given good corner placement a PC+ is pretty much ruler flat (in a mid sized room) to around 15-16Hz even in the 20hz mode
, so it's not like you should be missing much with the sub in that, its most efficient configuration.
Now that you know about the subsonic settings and the port plugs (this is recapped in the manual here: http://www.svsubwoof...CPlusManual.pdf
) ... by all means experiment a bit! There is no doubt that tuning your sub lower puts more stress on a woofer but within reason that variable tuning can and should be used to get the extension you want.
I think you will have a much better more accurate feel for the considerable limits of the PC+, and won't be bottoming out any more.
Get down to some nuts on accurate calibration runs with the different tunes of your sub (with your favorite demos run only AFTER you calibrate each time) and I think you'll only use "Love" to describe your SVS.
... of course it's hard not to love twins ;^)