Hi Alan,
Many of the Lii 15 baffle owners want to keep things simple and are just using them without any bass supplementation.
Your room is probably a bit larger than the rooms of most Lii 15 owners, so you are not getting the "reinforcement" of the side walls.
A ZROCK is certainly an option and would add some bottom end to the sound. The variable control allows you to dial in the amount you need.
Since the Lii F-15 driver goes down to 50 HZ, it's pretty easy to integrate a subwoofer with the main speakers and several of my customers have reported good results using a powered subwoofer.
One other option (but a bit more complicated) is adding the Lii Audio W-15 bass driver to the mix. It adds a wonderful foundation to any of the Lii Audio drivers, including the F-15.
Some info found
here.
You can order the driver from Lii Audio and have a local woodworker make a baffle for it if you don't want to build something yourself.
It doesn't have to be anything fancy and can be powered with an inexpensive plate amplifier with good results.
Happy listening,
Randy