What is with the 20Hz? If you are serious and want to maintain equal quality with your revels then you need a couple of good subwoofers like a couple of JL112's or 113's or a single high quality monster subwoofer (15" or greater) and you can get close 20 Hz. I'd budget around $10K to achieve this goal with less than 5 to 10% distortion at decent SPL levels. If you think there are any number of 20K speakers that can achieve 20 Hz with any kind of accuracy at decent SPL levels then I am afraid you have been reading to many reviews and manufacturer's wild claims...you need to look at sounstage distortion plots - most speakers are playing extremely high levels of distortion at 20Hz or the better ones are sensibly rolled off before 20 HZ.
Short List of the Best 20K Speakers
I'm looking to pickup a pair of pre-owned speakers that originally sold in the twenty thousand dollar range and can be had for around half that today. I currently own Revel Studios. I love them but want something that can go below 20 hz while still maintaining all the goodness these speakers provide. I've already heard and liked Dynaudio C4, Revel Ultima Salons (original) and Eggleston Andra II's. It's hard to get a real feel for what they sound like in your home with your equipment. Curious to hear from people who have owned or auditioned any of these. Also feel free to add any you would like to recommend. My amp is a Krell FPB600c. My room is unfortunately a 20' square with 8' ceilings. I have echobusters placed all over to keep the soundwaves from pretending to be pinballs in a pinball machine.