Interesting! I kind of see the SVS SB2000 graph (and Ricci's comment on what measurement would indicate a "tight and fast" sub) as validation of my theory. My first experience is with an SVS SB12-plus which was at its best when I had it close to the center of the room (minimizing room gain) and using the built-in PEQ set at 20Hz and maximum cut!. It was pretty good in this configuration, but it always struck me as a perverse irony that I paid extra for the built-in DSP and driver quality to be able to do ~20Hz flat anechoically, only to deliberately undo it!!! I bought the JL Audio e112's thinking I was getting the "ultimate" sealed sub only to experience the same issue (but moreso)! Gene seems to like his, but I assume that is because it is in his family/kid room and sees primary use in HT (or he has used DSP to tune it the way he wants).
At the same time, I know you have deep audio experience. So I don't know! I think your system is freaky!
Trying to think of explanations:
1) I assume this room is not large, but placement in the room may reduce room gain
2) Your LP may be in a null (but I suspect you would notice that when you move in and out of the LP).
3) There is a high pass filter somewhere in your signal chain that is removing the low frequencies that would show the room gain issue.
4) your content is absent low frequency content
5) you are playing at low levels such that the perceived loudness contour compensates for room gain
6) the DSP in your unit is not active
Another possible angle on this is that my problem was caused more by ringing induced by the DSP in my SB-12plus and e112 than the FR and your SB1000 being newer may have an advanced treatment of the DSP that significantly reduces the ringing (I'm not sure that the e112 is older). But that seems like a "reach" as I have heard of no such breakthrough and it would make sense for SVS to brag on it if a technology improved their subs significantly!
If nothing from this, I am at a loss to explain the difference between your experience and mine!