Hello,
I would go with REL T9i. It is front firing like the X and since it is the prior model you can get a good deal. $600-$700. Later you can pick up another since people will continue to upgrade. Don’t worry about the 20hz unless for HT. You will be happier with this. The T9 down fires for the active sub vs the T9i which front fires. Most prefer the front active woofer. I have the down firing version but I have upgraded to two JL Audio F110s and the CR1 crossover. I am going to incorporate the RELs in the back of the room when I have time to set it up properly.