Hi OP, Richard Vandersteen produces a great speaker and great value. One way to lift the performance is with subs but not just any subs. His own subs are expensive and I believe you can do better. One of his will just add bloat and boom to the power response and 'may' provide reasonable sound at one spot in the room.
noble100 has wisely suggested using multiple subs which is the only way to get truly good bass regardless of the main speakers' bass response. There is much to read on the subject, and I recommend you do. I could go on at length.
Perhaps take the time and look up Rythmik Audio. Brian Ding provides servo sub solutions that can be bought ready to go or as kits. Also GR Research. I have found Rythmik more helpful. The kits are a bargain and the dedicated plate amps are configured for fine adjustment
Servo bass is the best I have heard and Open Baffle subs the best of the best and would be a perfect match for electrostatics or planars. Now that I at last have a house I will be building OB subs to partner my Tannoys, also OB.
At the 2019 AXPONA, which unfortunately I did not attend because I live in a different country, many show reports mentioned the OB servo bass by, I think, Sound Insight partnering GT Audio Works planar speakers. The full towers, 6X12" drivers per side run about $28K a set. Read the rave reviews. Servo bass often awarded best of show at numerous venues.
The parts for 4 drivers and 2 plate amps excluding cost of wood, glue and finish for 2X 12" servo per side when I looked a few years ago was under $1500! These 2 will easily fill my 16 x 22 ft. room. The OB drivers are 16 ohm and 3 per side paralleled is popular. Then add in 1 or 2 more to balance the room. I helped a friend build and install OB subs and 2 sealed subs in sand filled enclosures because he watches movies a lot. Slamming, tight and awesome. No bloat or boom. Servo precision Use something like Omnimic from Parts Express to measure and remove guesswork in positioning the subs. Its needed to optimise room treatment which is essential