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

Or Rythmiks.

@4425, to answer your earlier question 20K in parallel with 300K is 18.75K, and 20K in parallel with 1M is 19.6K. I doubt that there would be a noticeable sonic difference between 20K and 18.75K. However I suspect that 20K already represents a slight compromise (I recall that the specs for some older ARC preamps say "60K recommended; 20K minimum"), and given the caliber of the equipment you are contemplating I would not want to introduce any compromises, however slight they may be.

50K in parallel with 300K is 42.9K, and 50K in parallel with 1M is 47.6K. So the JL Audio Gotham or Fathom subs I referred to should be fine in terms of impedance compatibility, if (and only if) connected single-ended.

Good luck. Regards,
-- Al

very helpful and much appreciated. 

I have driven a JL f212v2 with LS28 using its balanced outputs without any issues.

I recently upgraded my pre to Ref6 and now considering the REL 212/SE. I used to own a pair of B1’s and IMO, REL integrates much better than any other sub in a 2 channel system.

The plan is to drive 212/SE using its high level input from speaker output terminals of my GS150 amp.

Thanks for your input, Lalit.  Well, the JL F212 v2 has a balanced input impedance of 10K.  Certainly not an ideal load for the LS28 (which has the same 20K minimum load impedance spec as most ARC line stages and preamps).  And the paralleled input impedance of the power amp would only make that worse. 

I would consider your statement of "without any issues" as indicating "without any apparent issues."

Best regards,
-- Al