Connecting a sub and using headphones

I have a NAD 375BEE Integrated amp running 2 sets of speakers. A set of Paradigm S4 V2's and a set of Triad silvers. I have a Definitive Powerfield {I know it's old} 15 sub woofer (on spikes)with a line level signal coming from the pre-main out to main in jacks on the NAD. This set up works and sounds fantastic and keeps the amp from having to drive the low end (60Hz<)that the speakers aren't capable of handling accurately anyway. I call it a win win. The problem is when I want to use my headphones after my wife has gone to bed I switch off both sets of speakers but the signal still drives the sub. Is there another way to set my system up or add switching box to eliminate this problem. There is no sub-woofer output on this particular amp. There is a tape monitor loop I can use but I don't think the subs volume will vary as the volume on the amp changes if I use the tape loop. If I am not on track with this line of thinking I am sure someone will correct me
Any advice you can offer will be helpful!