Ed,
I've made the PB13 mobile this evening and moved it about 3.2 meters out of the corner.
As a "quick & dirty" test I hooked on my portable PC and put the volume very low (safety...) and also the gain on the PB13
I then went up to 2/8 gain and started rising the output signal on the PC (headphones-out) and I had the X-over engaged to filter out unwanted higher freq.
Tested with some high quality, low freq MP3's ... the muffled sound that came out together with the bass where quite ... :raped:
...holy cow...

the difference was already day and night. Now I felt the bass and things started to move (some drinking glasses 8meters further seemed to be shaking...)
So basically I think :
- OR the signal from my Onkyo was way to low
- OR I have a serious room problem that is "better" at its current location shifted roughly 3.2meters away from the corner. (room is 12.2meters in length, so that's near 1/3 of the room)
The funny thing is I did the Audessey run yesterday with the sub in the corner and the gain about halfway.
Audessey configured the subwoofer at -15dB.
Even after changing manually to -3dB or even 0dB is still was not the impact that I felt now ...
So by changing that sub-signal to -3dB or 0dB (heck, even tried +3dB if I recall well) no real improvement was there so I guess it is really caused by placement....
Let me fiddle some more and I'll do a new Audessey run tomorrow and see what happens...
Hopefully things will improve much...
Thanks for all comments so far.