This is very true.
Just recently I got myself a HD650 as I wanted to make use of the headphone output of my Marantz PM8006 and I am less than inspired.
The sounds falls way short of HD650 fame.
I later learnt that PM8006 doesnt have a separate headphone circuitry and it just steps down the speaker output.
However, going by this review your amp is a fairly competent one in this regard.
ARCAM FMJ A19 INTEGRATED AMPLIFIER
headphone.guru