Dual SVS Subs with Mcintosh C2500

I have dual subwoofers too. I learned that my Audio Research preamp, while having 3 sets of Main Outputs both RCA and XLR, has a minimum input impedance requirement of 20 kOhms. Since the SVS subs have an input impedance of 22 kOhms and my Pass Labs amps also have 22 kOhms impedance that puts a load on my preamp mains at < 10 kOhms. This can impact the quality of the sound of the preamp. I also believe it puts a strain on the power supply of my preamp- specifically the 6550 power supply tube.

Check to see the minimum input impedance requirement of your C2500 and also the input impedances of your MC501s. Each amp/sub is in parallel so the impedance is the sum of the reciprocals. (1/Z1+1/Z2=1/Ztotal)

My solution was to add an op amp in the subwoofer lines. The op amp has a 100 kOhm balanced input impedance so in parallel with my Pass Labs amp the load on my preamp is now 18 kOhms. This is within an acceptable margin for my preamp. And since the op amp is working in the limited bandwidth of my subs I detected no degradation in sound quality. Of course this op amp is rated for the full audio bandwidth.

Of course you can put the subs in series with your amps.  That is the simplest way.  The SVS subs accommodate pass through of the signal.  I chose not to go that route because I already had 5 meter cable runs from my preamp to amps.  That solution would have cost me $$$$ to replace those cables since they could not reach my subs and would have been much too long to go between my subs and amps.