I agree with Mofimadness. In fact, I am doing the exact same thing with my rig. If youy check some of my threads, you can see some of the technical issues that I had to overcome, but some of these issues could be unique to my set up.
By way of quick summary, I have an ARC Ref 3 pre which has two main outputs. Main 1 connects to my power amp. Main 2 connects to my self powered sub. It appears that your situation may be a little simplier than mine because it looks like you're working entirely with SE/RCA hook ups. In my case, one hook up was SE and the other balanced.
Now . . . as Mofi suggested, check with your preamp manufacturer to see if you can use a Y connector to sum the left and right channels for the preamp output feeding the sub. If not, you will need a crossover to do that for you. One problem I had, which again may be unique to my set up, was that I had to show my Ref 3 a combined load that was not less than 20K Ohms. The problem was the two loads (i.e., Main 1 - amp and Main 2 - sub) that I presented to my Ref 3 summed to be less than 20K Ohms. So I had to purchase a custom made impedance buffer that summed the Main 2 channels and raised the load presented to the Ref 3 to make everything work properly.
So . . . the take-away from this post is: (1) see if you can sum the channels into mono for the sub with a Y adapter. If not, look into using a cross-over that will sum the channels into mono for the sub, but leave the other output in stereo for your fronts. (2) make sure you don't overload your preamp by presenting too low a combined load. In both cases, discuss these issues with your preamp manufacturer.
BTW: just a suggestion, before I switched over to the custom impedance buffer, I used to sum my Main 2 outputs with a Paradigm X-30 controller. Check the Paradigm listing here on A'gon. The X-30s come up for sale from time to time and sell for roughly $150, plus or minus. The X-30 will sum your channels without "shorting" your preamp, and also help fine tune blending your sub with your fronts by giving you the flexibility to adjust LF rolloff, loudness and phase. FWIW
Good luck and let us know how you nake out.
By way of quick summary, I have an ARC Ref 3 pre which has two main outputs. Main 1 connects to my power amp. Main 2 connects to my self powered sub. It appears that your situation may be a little simplier than mine because it looks like you're working entirely with SE/RCA hook ups. In my case, one hook up was SE and the other balanced.
Now . . . as Mofi suggested, check with your preamp manufacturer to see if you can use a Y connector to sum the left and right channels for the preamp output feeding the sub. If not, you will need a crossover to do that for you. One problem I had, which again may be unique to my set up, was that I had to show my Ref 3 a combined load that was not less than 20K Ohms. The problem was the two loads (i.e., Main 1 - amp and Main 2 - sub) that I presented to my Ref 3 summed to be less than 20K Ohms. So I had to purchase a custom made impedance buffer that summed the Main 2 channels and raised the load presented to the Ref 3 to make everything work properly.
So . . . the take-away from this post is: (1) see if you can sum the channels into mono for the sub with a Y adapter. If not, look into using a cross-over that will sum the channels into mono for the sub, but leave the other output in stereo for your fronts. (2) make sure you don't overload your preamp by presenting too low a combined load. In both cases, discuss these issues with your preamp manufacturer.
BTW: just a suggestion, before I switched over to the custom impedance buffer, I used to sum my Main 2 outputs with a Paradigm X-30 controller. Check the Paradigm listing here on A'gon. The X-30s come up for sale from time to time and sell for roughly $150, plus or minus. The X-30 will sum your channels without "shorting" your preamp, and also help fine tune blending your sub with your fronts by giving you the flexibility to adjust LF rolloff, loudness and phase. FWIW
Good luck and let us know how you nake out.