How do I connect an outboard Headphone Preamp to my 976? I want to bypass the op-amp in the 976 and introduce a higher quality preamp since I listen to headphones a lot.

I want to be sure that the main speakers are "off" when I listen on headphones.
Outlaw Model 976 AV Surround Processor
Outlaw Model 5000 5 Channel Amplifier
JBL Studio 590 Left and Right
JBL Studio 520C Center
2 Martin Logan 500 Dynamo Subs
Polk 265RT 3 Way Surrounds (in wall)
Panasonic TC-P65VT60 65-Inch Plasma
Panasonic DMP-BDT500P Blu-ray
Technics SL 1210 GR (2019)
Ortofon Bronze 2M; Ortofon Headshell;Emmeline Nighthawk Phono Pre-Amp by Ray Sammuels
Schitt Loki
Drop THX AAA 789 Headphone Amp
Blue Jeans Speaker Cable Belden 10-gauge 5T00UP (Front L&R and Center)
Blue Jeans Interconnects LC-1's
SKW Audiophiles RCA Cable
Furman M-8X2 Conditioner
Panamax M4300-PM Conditioner