The Avantgarde horns are not lacking in bass! However, the is a 50Hz null at my listening position and the REL S-812 sub helps to remove the null and smooth out the bass. I have the sub sited behind the listening spot. When you just listen to the sub you would not imagine it does much. However, if you switch it off whilst playing you immediately lose quite a bit.
I really wish REL would have an APP like SVS do as it is a pain when setting up to keep getting up to adjust the volume/cutoff.