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

@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 

@4425 

After reading this document at the the ARC website (http://www.audioresearch.com/ContentsFiles/The%20Thorny%20Math%20of%20Impedance(0).pdf) and doing the math, I realized that my REF6 was unable to properly drive both my Martin-Logan CLXs (from an ARC 150SE) and the M-L BalancedForce 210 subwoofer, when using the single-end inputs (10K input), as I was doing.  I was a bit shocked, to be honest.

This was discussed with both M-L and ARC and it was determined that I would be fine using either the balanced inputs (30K input), or the high-level inputs from the ARC 150SE (20K input).  I opted for the former, and purchased a set of Sub3 balanced XLR sub cables from Morrow Audio, to replace the Audioquest sub cables I was using.  There seemed to be mild improvement, but it was not striking.  Probably more to do with my piece of mind.

Either way, the M-L BF 210 is a spectacular subwoofer.  The ability to match the crossover to the main speakers exactly for your room and listening position using the Perfect Bass Kit (PBK) that is making a custom filter for the bass to correct for room nodes and antinodes is just amazing.  The runs up and down the neck of a stand-up or electric bass are clear and distinct, with each note at approximately the same volume.  No "in and out" of the notes, or booming, that I have heard in every other system that actually has bass.  To say this enhances listenability does not do justice to the improvement.

I am smitten, as you can tell.

Good luck on your search,

-jim