The stock jack is a switched jack. The 2.49K resistors keep the output end of the 100uF coupling caps at DC ground potential. Without them you would have about 100V DC sitting at the headphone jack waiting for you to plug your headphones in, and you'd get a big pop and a spark, and probably one or two blown headphone drivers every time you plugged the headphones in. The switching function of the stock jack grounds the outputs when no headphones are plugged in so that this voltage can't build at all when no headphones are plugged in.
The stock jack is also insulated from the chassis plate, and if you use a jack that automatically connects the sleeve to the chassis plate, you will introduce a ground loop and the chassis plate will become part of the signal path, and this would be a serious downgrade in amplifier performance.