In general, when a headphone amp is on should it have a load:Specifically, headphones plugged in as is usually required of speaker amps having speakers connected?

Huh? Sorry I don't really understand your question.

Do you mean to ask whether a headphone amp can be turned on without headphones plugged in?

I don't personally leave an amp on without something connected for more than a few seconds.

With SS amps I don't think it's an issue, with tube amps I've been told (but can't really explain) that they should not be left without a load for very long.

I don't personally leave an amp on without something connected for more than a few seconds.

Why is that so? Unless you mean simply that there's no point turning on the amp without a headphone.

Edit: just saw n_maher's reply

Interesting... Why is that so? In fact I heard from some friends of mine that they usually leave their amps on for a while before using to warm up, and because they own a few cans they usually just let the amp warm up without anything connected.

I don't particularly have a reason. But I don't switch between headphones often.

Some OTL tube amps don't have a built in load (say 10k or so) after the

output capacitor. On an amp like this, if you leave the output disconnected

for some period of time with the amp on, the cap discharges, then when you

plug in the headphones there is a signifcant voltage spike (say 100 volts or more)

which can definitely do damage to the heaphones.

Singlepower and woo audio amps are known to have output load resistors.

Transformer coupled amps are unlikely to have any problems.

Why would somebody even omit the resistor to ground after the output cap in the first place? Is there supposed to be some kind of benefit to doing it that way?

I think it changes the frequency response. Minutely, admittedly, but it's "old school".