hi Alex,
I just came across your post. I don't think setting the SW dB to 12 will solve your problem if you don't know your combination well.
I believe the problem isn't at your AVR side. I believe it's your equipment isn't match well.

I've done many installation with 1909 AVR and I never get over 5dB on the AVR side while the subwoofer level still stays below 50% par.
I'm not sure what kind of sound you are expecting your SW to give you, but from what I see, you probably want to have some super low level bass that you can't archive and that's why you max out your AVR SW side.

Check your Roger subwoofer frequency.

IF u can answer some of the questions below, i'm sure there will be forumers who can give u further advice.:

1) what's the dimension of your listening area? Room or Living room?
2) what's your roger SW model and size?
3) what subwoofer cable are you usnig?
4) where is the subwoofer located?
5) front and rear speakers model
6) AVR level for each speakers.

good luck.

you probably give the wrong image of the Y cable.
Should have a RCA female port rather than mini jack port.

anyway, i think Y cable will not help alex.
He has already max out his sw level at his AVR side. I would probably think that alex's expectation might be different from most of us. I would assume alex is expecting high treble and powerful bass like his car audio.

for denon avr, u can increase the tone control for bass n treble but usually we set it flat to get the most natural sound from the movies.
for SW, we also set to a rational level to match our movies characteristic.

for your usher speaker, i would say it's a power hungry speaker based on the sentivity given by alex.
from this point, i would say it's a mismatching combination that caused the problem alex is having.
To resolve alex's problem, i think one has to be there on the spot to tell wat alex is expecting to hear.

apparently, alex could be having phasing problem on the speakers too.

n hand writing on user manual??? could it be a used unit ? where did u got it from?