This evening I have been trying out a Subpac S2 (with normal headphone listening, using HD650 and HD58X). Initially I was very, very disappointed with it. I could not get it to integrate well, even after splitting the signal paths of the headphones and the Subpac to enable more precise balancing, and using a graphic equalizer to limit the range of the Subpac to below 60 Hz or so. I have the feeling the Subpac is distorting, creating harmonics that are not present in the music. It reminds me of big cheap speakers with flimsy cabinat panels resonating with the bass notes. Also I think there was a timing/phase problem, like the Subpac was running behind.
Now I managed to get some improvement but with a lot of effort! I connected the front-pre-outs of my Yamaha RX-V771 receiver to the main-in of my Yamaha AX-592 (and connected the headphones to the latter, my front speakers are also connected to the AX-592 so I can switch them off while using the headphones). I connected the sub out 1 and 2 of the RX-V771 via the graphic equalizer to an input of one of my 2 Yamaha RX-V750 receivers and connected the subpac to it's headphone output.
Now on the RX-V771 I can change the subwoofer crossover frequency to control the range of the Subpac (low end goes to both front and sub), and delay the main fronts (and hence the headphones) compared to the Subpac (by increasing the distance of the sub). (I don't use the graphic equalizer now, but I can if I want to try more adjustments.)
Although better now, for my taste it still is a strange experience to use it. Somehow the contribution from the Subpac still feels disconnected from the sound over headphones to me. Of course I tried to follow the often heard advice to turn down the Subpac to the point that it is barely conciously noticable. But as soon as it is noticible it feels disconnected, and as soon as it is not noticable I can not feel any difference with not using it. One of the things that makes it feel unnatural to me is that it works so localised on one part of the body, the back.
So I think I am going to return it.