Curtis is right, without filtering it will not be such a great sound in the headphones. However, use regular headphones and put a series 32 ohm resistor on the speaker output to the headphone jack. Wire the jack so it disables the speaker when you plug in the headphones ie no need for an extra switch. Neutrik jacks make it real easy.
We have added external speaker jacks to the Frontman amps and the headphone jack mod is nearly the same with just the added resistor.