Sounds like (very) high output impedance (I guess the Superlux cans also play quieter at same volume setting?), maybe even an oscillating amp. Not the cans in any case.
What sort of amp is that?
The current version with the bunch of BC337/327? Might be oscillation then after all. The current stage can, at least in theory, oscillate. I suggest you try adding something like 220 ohms in parallel to the load, and/or an increase in output series resistance (R21 - with the headphones given you should easily be able to afford 10 ohms).
i think the panic is over 
i have always had a slight mains hum plugged up to my oppo bdp93 and even more so if i have anything else plugged into the same mains line, until tonight.
step up the RPi with IQaudIO DAC+ with phono sockets removed and connected direct to the in of the Sapphire.. complete silence no matter what i have turned on in the mains line.
oohhh and the RPi and DAC+ sounds blooody great through the Sapphire.
thanks for all the help and its much appreciated.
i have always had a slight mains hum plugged up to my oppo bdp93 and even more so if i have anything else plugged into the same mains line, until tonight.
step up the RPi with IQaudIO DAC+ with phono sockets removed and connected direct to the in of the Sapphire.. complete silence no matter what i have turned on in the mains line.
oohhh and the RPi and DAC+ sounds blooody great through the Sapphire.
thanks for all the help and its much appreciated.
So it was a ground loop issue after all. I take it your amp is built as an IEC Class I (safety earthed) device with audio ground connecting to chassis somewhere, and same applies to the Oppo?
You would have had no issues with either an amplifier that's floating on the Oppo (which uses a SMPS, so the PE connection makes heaps of sense), or a floating source on your Sapphire. Combining both is asking for trouble.
Really, the modern A/V playback system tends to be a mess of ground loops. Balanced interconnections or equivalent ought to be mandatory by law. HDMI uses differential signalling, so that's OK (in theory, but not always in practice). Toslink uses optical transmission, and coaxial SPDIF would also be OK if at least one side bothered to use the recommended transformers (which is not necessarily the case, unfortunately). But classic unbalanced audio interconnects are a big FAIL.
You would have had no issues with either an amplifier that's floating on the Oppo (which uses a SMPS, so the PE connection makes heaps of sense), or a floating source on your Sapphire. Combining both is asking for trouble.
Really, the modern A/V playback system tends to be a mess of ground loops. Balanced interconnections or equivalent ought to be mandatory by law. HDMI uses differential signalling, so that's OK (in theory, but not always in practice). Toslink uses optical transmission, and coaxial SPDIF would also be OK if at least one side bothered to use the recommended transformers (which is not necessarily the case, unfortunately). But classic unbalanced audio interconnects are a big FAIL.
the headphone amp is a 2 box. one box is the transformer and rectification and the case is connected to PE. the second box is the headphone amp boards connected via an umbilical of only the rectified power, case in the headphone amp is connected to COM with no PE.
with only the headphone amp powered up there is a really bad hum then when i switch on the RPi its an inky black silence and i mean silence no hiss no hum nothing even at full volume on the RPi.
with only the headphone amp powered up there is a really bad hum then when i switch on the RPi its an inky black silence and i mean silence no hiss no hum nothing even at full volume on the RPi.
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