Which subwoofers have at least a 20k input impedance as required for ARC preamp

thanks al. very good info. i need to do some studying. have passed on arc preamps in the past bc of the dicey nature of the pre driving a main amp plus a subwoofer. 

the nitty gritty. i’m interested in a ref 6 to drive a 20k sub plus either an arc amp w a 300k ohm input impedance or a Dag amp with a 1M? input impedance. Would either work well? Also the 50k JL might work better? Just not proficient enough to decipher. I hope i got the specs correctly. what do you think al? 

very helpful and much appreciated. 

Thanks for all the input. Actually the issue is very complicated after I spent about an hour on the phone w ARC yesterday. In fact we didn’t really come up with a comfortable combination of amp(ref 75se) and sub.
Running balanced into the amp and unbalanced into the sub is bad regardless of the subs impedance spec such as JL’s 50k spec.
Unless the sub is true differential balanced it’s really single ended. Don’t know about the JL.
The solution might be balanced out to a sub with true balanced inputs and a reasonably high input impedance. Good luck on that one.
To try to understand all of this a potential ARC buyer should call them. They were working the math and it was complicated with no real conclusion.
Obvious answer is ARC preamps are not really designed to drive subwoofers.
Almost forgot. He thought the REL speaker level connection might work.
Im sorry that I’m not technically astute enough to better explain but I personally won’t be buying an ARC Ref pre due to this and the fact that ARC indicated that the 6550 tubes in the power supply of a Ref preamp could be very problematic should one fail in a bad way. Then there’s the amp output tubes.
Regardless of what is posted here there is apparently is a lot of shipping back and forth to ARC for service. For me there’s too much great SS now to suffer the grief unless you’re lucky.
My apologies for the length of the post.

Al: I wish I had your knowledge. I’m too simple on audio and don’t want to go a little funky or compromise in any way if I was buying uber ARC products. That’s my problem.