Short List of the Best 20K Speakers

The V5As are excellent and extremely good bass performance, especially with the ability to "tune in" thew bass to the room, not sure even they do 20Hz, not do I think that should be the goal of a speaker. It would be easier to answer the question if the OP just wanted better bass performance than he is currently getting, which could also be done with the very same speakers and room treatment. For "extreme" low bass, it does seem to me the subwoofer approach would be best because it is awfully hard and or expensive to get that kind of bass from a speaker and not be compromised in other ways, especially at 20K. I suspect your 5As would provide world-class bass in most rooms, for most music.

1st, Enter 'room mode calculator' in Google. The lengths of sides being 2 1/2 times the height is awful, not to mention the room being square!

2nd, In this case the room is really your enemy. Spend some time and money on the room. If the total budget is 20 large, apportion some of it to getting the acoustics fixed. Do that at the same time you install your stereo.

Finally, 'Add a sub' is probably not advised at this point. When I did as I suggest in point #1, so many frequencies popped up that were even, or near even multiples of 20hz that I can only imagine the room as a big, vibrating echo chamber.

You can put 20 large in speakers only, which is 2x what I have in my entire, reasonable sounding system, add another chunk into electronics and a GOOD source and still have unsatisfying sound. Before you get on this merry-go-round, consider your room.

I'll bet your EchoBusters are some of the best money you ever spent! Low Frequencies are the absolute worst!

Spend the money on improving your listening environment if you really want to (it's a free world), but real 20Hz with decay for ambience is going to be extremely difficult with the dimensions of your room. And why? Most music has little to no first order 20Hz cycles. Are your trying for "Dafos", organ music, HT action movies? There's so much good music out there without 20Hz and trying to keep this particular parameter is going to be a money sink where you'll just piss it down a hole.

I've done it; Double Entecs, Maggie IV Bass Panels, Pow Wow. I never got it right to my ear and even turned those subs off during all my regular listening. Honestly, IMHO a well balanced music system with 20Hz bass is not in the cards for $20K.

Coincident Pure Reference!!!!!!!!!

I just auditioned (again) the Devore Fidelity Reference Silverbacks. It is getting more and more difficult to not pull the trigger on these incredibly musical and beautiful full range speakers. They sell for around $17000 new but outperform a lot of speakers costing much, much more. Definitely worth a listen!