Dual SVS Subs with Mcintosh C2500

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.  

There is no one proper way, but I would just use splitters out of the preamp to feed the subs. Left and Right channel. 

So in layman terms, use the unused Main balance. This means use the XLR/ RCA splitter to subs?! Below is input impedance specs for preamp and amp..

Preamp shows; Input Impedance- High Level, 22K ohms unbalanced / 44k ohms balanced

Amp; Input Impedance- 10,000 ohms Unbalanced Inputs / 20,000 ohms Balanced Inputs

The only issue I've had was to many splitters on the same output. I use a Cary SLP-05, C2500 and a C22 (vintage). The C2500 (with some sub plates) the signal is to weak to run multi signals on the RCA side of the C2500. The XLR side of the same output didn't suffer the same affect that the RCA did. I used all 6 XLR outputs and the unswitched pair went into a bass management system. It contoured 280hz and below with two column of 8" drivers (16 total) and 4 double servo units. 

I recommend  using a left and right signal from the preamp too. Not just 1/2 of the signal. The bass is laid down in stereo and sometime stereo + (they will image a center channel from 100-300hz). Below 60-80hz  the signal is in stereo though it produces a omni perception of MONO. It's not, but for all intents and to simplify it, our brain says "everywhere". The schematics say otherwise..

The C2500 has adjustable gain on the line in. It's to contour the loudness (gain) between sources. I'm not sure on the output except in HT bypass it is set to a certain gain..

Use your Main balance output to your MC 501s, use output 1 & 2 for your subs.  Make sure to use the R channel of output 1 & 2 interconnects for your right sub and do the same for the left,  I use this configuration with my 2 JL Audio E112 subs using a Mac C50 preamp sounds fantastic.