It depends on what source is being played at the time. If it's a 5.1 source, it makes some sense: the LFE track would be redirected to the mains with the sub off. If it's a 2.0 source, that's a little odd.
If you have a stereo recording playing with large speakers, the LFE sub will get nothing - because your speakers are set to "large" and thus don't need a sub to assist them. Keep in mind that with large speakers, the LFE sub will only get a signal if a x.1 source is playing.

Here's one thought... set the fronts and center to "small", use a splitter to divide the subwoofer output four ways and run it to the LFE sub and all three front subs.

That still leaves the other subs up in the air, as I don't think you can set the surrounds to large when you have small fronts (could be wrong there). The issue there, however, is that you are not likely to get meaningful bass sent to the surrounds when using 5/7 Stereo - other sources simply don't typically put deep pass in those channels because so few systems reach much below 80Hz in the surrounds.

One other issue - both your Polk PSW10's and your Velodyne CHT8 roll off around 35Hz - which is just a few Hz lower than my old Paradigm towers reached without a powered sub. The subs in your DefTech towers actually go deeper, although I wonder about the reality of DefTech's specs (I'd like to know how many dB down they are at 15Hz, as I doubt it's just -3dB or so). Those three little subs will not get the "best 'heart-pounding', deep, clear bass" that you might be looking for. That reinforces my thought about hooking the DefTech subs up to the 950's subwoofer output. Certainly I'd lean toward going with small rears and let the 950 steer low frequency elsewhere rather than hook the Polks or Velodyne up via speaker cable.