I would hold on to that $300 for now and wait till you can get something more substantial to fill that space. IMO you will be very happy with the 3.0 Focal setup for now sans sub. Since like you said you know little about subs, you won't know what you're missing, likewise if you spend the $300 on a sub IMO you will still not know what your missing, so save the $. The new SVS's look appealing, but I think you may need two
Save up for the PSA XV15, that will awaken your room, I know
POWER SOUND AUDIO — XV15