Home theatre in basement is completed. Bass comes from a PB12-Plus/2 in addition to the built-in subs from Def Tech L/C/R. The sub is set at 9 o'clock (with off being 6 o'clock) and is set for 20 Hz. Setting the gain any higher gives too much bass, but it is heard and not felt.
Try using just the SVS and see what happens.
I'm assuming this is because it is in the basement which is carpet over concrete. Am I assuming correctly or do I just need more subs?
No, carpet will have no appreciable absorptive effect on a subwoofers passband. The concrete, being unyielding, will make the room modes far more energetic than if the room had more flexible construction.
I think your problem is not with the number of subs, but rather with their positions.
Is it just not possible to feel bass on a concrete foundation? I really want to feel the bass but it is just not happening.
Bass will be more pronounced on concrete because it is acoustically reflective, the problem comes in because room resonances will be far more pronounced, and then some frequencies will be much louder than others, and bass will sound (and feel) bad.
Bass covers four octaves, it is very important to know where in those octaves the problem is for a solution to be reached, as was suggested, you
really need to find out what the in room frequency response is.