You and
@Gasp3621 know your stuff so I don't think there's much I can add here really but, assuming no hardware faults, logically it can only be either:
1. The OP is sitting in the mother of all nulls. While unlikely I do have one such seat in my (single sub) lounge system and you could almost think the sub wasn't on so worth checking other spots. Obviously, that's not my seat in that room!
2. Something's wrong with the AVR's settings. I think you've covered this really well but the only thing I can think of that my Yamaha has that I don't think has been mentioned already is the "LFE level". On mine the (factory) default is 0dB but this can be cut by up to 10dB. The idea is you use this is your sub can't handle 30dB peaks at your listening level. Frankly, I'd be tempted to do a full factory reset on the AVR if this issue isn't fixed.
3. The AVR isn't receiving a multi-channel bitstream digital signal (i.e. no LFE) from the playback device. This is easily done as PCM is often the default output on source devices. What does it say on the AVR's display? To me this is the most likely culprit.
4. The sub is woefully inadequate for the room's volume. Even with standard 2.4m ceilings, a 10m x 6.5m space is going to be at least 5,500Ft3 in volume so this is quite possible. That volume is even above Audioholics' Extreme Room size and would take a very powerful sub to pressurise to loud listening levels - PB-3000 is SVS's lowest sub recommended by AH for such a room. That said, if the MLP is fairly close to the sub then I would still expect some decent impact so this is unlikely IMO.
I'm afraid that's all I've got for you. Of course the OP could also contact SVS direct via phone or email - their Customer Service guys have seen everything before so are amazing at bottoming out issues like this.