I don't know how you term the sub average.. Where you had the demo, was it a single sub?..
I had dual Taga Platinum Sw10 v.2 for abt 18 months time in my room and did measure them, and it can reach abt 22Hz in my room.. What matters is the SPL, which any budget sub would struggle to reach.. That is a limitation of any budget sub.. So why blame saying it is average..
There is no way to know how good a sub is until you measure it (or) you you sit very close to it.. Ears can't judge the low freq in the room accurately..
Even if you have an sooper expensive sub, moving a feet from your listening position can change the response.. Assuming atleast 2 ppl sit in a room to watch movie, then dual subs will defnetly have advantages over single sub..