amp pre amp matching

joes44, In the manual they stated that >60k/<100pF is for optimal results, but >20k/<1000pF is minimum requirement.  I looked at the schematic and it appears that in addition to Mosfet output stage there is 5uF output capacitor.  This capacitance with 20k load would give you about 1.6Hz limit (below specified -3dB@5Hz).  So perhaps they are concerned with output loading, since maximum specified output voltage is 50Vrms = 70.5V peak.  It would cause 3.5mA@20k peak current that Mosfets might have difficulties with (some nonlinearities).  It is hard to imagine, but I cannot see any other reason for 60k requirement.  I wouldn't worry about it.

In the manual they stated that >60k/<100pF is for optimal results

That's a low load capacitance... as a rule of thumb coaxial interconnect works out at about 150pF a meter so you might want to factor the cable length in. BTW low cap coax is available, just not common... I use 55pF/m coax from by turntable to phono stage.

I have an Audio Research SP9 MkIII pre-amp. I use an Audio Research 100.2 amp which has an input impedance of 150k ohms single-ended/ 300k balanced. I am using a 20’ ARC IC between the pre-amp and amp.

A few years ago, I bought some System Audio Mantra 50s together with a JL Audio Dominion 108 subwoofer. I hooked the subwoofer up with the line level inputs (as recommended by JL Audio). I was pleased with the sound after tweaking the subwoofer settings.

Recently I bought a pair of Spendor S9s. I used the subwoofer at first but later unplugged it from the AC power since I felt the speakers did not really need it. Recently I also unplugged the interconnects to the subwoofer. There was a substantial improvement in the sound after this 2nd change; fuller and more dynamic.

It was then that I started looking into input impedance. The flyer on the JL Audio website says that the line-level input impedance on the subwoofers is 20k ohms (although the manual says 10k ohms).

If the impedance is 20k then hooking it in parallel with the amp with 100k impedance gives an effective input impedance of less than 17k (below the recommended impedance for the pre-amp). If the subwoofer’s impedance is 10k then the combined impedance is about 9k.

When I rotate the System Audio’s back into the system I will hook up the subwoofer using the speaker-level connections. Interested to see how it affects the sound.

I am looking at getting another amp since the Spendor S9s could probably use more power. The input impedance of the amp is definitely something I am looking at.

250 ohms into 22K is perfectly fine, on paper. The < 100pF recommendation is odd. A lot of audiophile interconnects will exceed that. And by a LOT, if your doing a long run to the amps.