I can’t hear the Difference in the change the low pass filter or the volume on my SVS subs

I only have one sub, SB-2000 Pro, and I keep the volume up as high as it will go, 0db.  I can clearly hear a difference on the low pass filter, especially between 40Hz to 60Hz.  I leave it around 50Hz and it fills in nicely.  You can adjust the filter in real-time with music playing.

How are you feeding it, line level inputs? 

I'm not sure if my positioning is unique or not but I'll describe it:

I'm set up in my living room.  My couch and love seat are in an 'L' configuration and the couch is the focal point.

An end table sits between the couch and the love seat.  I have the sub under the end table and it points away from the listening area making it a source for background and filling in.  There's a caveat however.

For the sub to sound its best and to 'disappear', you have to be at the focal point of my listening area.  I have the speakers tilted a slight amount.  If you leave the listening area, then the sub no longer disappears.  As a matter of fact, its presence  is very evident.

I mentioned the sub points away from the listening area so behind the love seat, you have a birds eye view of the sub's grill and it is very noisy back there.  The only one that does not seem to mind is my dog lol