How difficult would it be to add a headphone out line to an amp?

Headphones are shitty anyway. Just play it regular and loud.

Just buy or build a headphone amp and plug it into the "pre out" jack. Or you can even try plugging your phones directly into it; I don't know if it'll be able to drive your headphones very loudly though.

>>934888
What amp do you have?

If it's a good solid tube amp I'd rather get an isolation box and mic it up then use headphones on the interface.

>>934895
This op. This.

>>934888
Fuck, just build an h-pad. Google it. An h-pad will dump from 0 to as many dbs as you like. Most bookshelf stereos do it this to feed headphone jacks. Manufacturers just pad the output to the speakers by 40 dB and send it out to the headphone jack.

Might be a dumb suggestion, but couldn't you just use a 1/4" to headphone connection adapter. This is what I use on my bass amp.

>>934892
Try this. A preamp should definitely drive the headphones.
Keep in mind it may sound overly bright, even very harsh.