You have at least two options if you want to use bigger headphones. One is to get a pair of powered speakers with a headphone jack, there are lots of these on the market. Another is a dedicated headphone amplifier, for example the Edcor 2020.

I did finally buy one of these amps. I was tickeled most by this from the operating instructions.

"We know you are going to look inside. Please unplug the AC adaptor prior to removing the 4 small screws on the back cover. Now peer inside and admire the creative design and the solid construction...."

I just find my self wishing they hadn't used the same diameter jack for the AC input as is found for the audio input. (bad things happen when you plug the ac into the buss input) I'm waiting now for the digi-key order with a couple of new op-amps. Those chips are all socketed so, after I replace a couple of $0.60 parts, it will live again. I'm not impressed that the chips are socketed, the ac jack should not have been compatable with the audio inputs.

Glenn