Why a headphone amp

Most modern headphones will be driven to ear-shattering volume levels by inputs that are not much more than a milliwatt (0.001 watts). Your 300 watt monoblocks, although less powerful than that when working into the relatively high impedance of most headphones (compared to speaker impedances) would still be WAY too powerful, and would also provide voltage gain that would force you to use the preamp's volume control at just about the bottom of its range.

There are a few preamps that are suitable for driving many headphones directly from their main outputs. Although most preamps are not suitable either (unless they explicitly incorporate a headphone amp function), because of output impedances that are too high and/or other reasons.

Regards,
-- Al

There are adapters that can be used,but why would you? The cost of a decent HP amp is not much more.

There are some people that use a poweramp for headphones. The amps are usually low power tube designs, not high power SS.