I just got a pair of SH pxc 550's and I'm having some issues whilst using them as a live monitor through a mixing desk. I am using a pretty basic pre-amp which seems to work with every headphone/ earphone that I have tried.
However, I seem to be having issues with sound quality and volume when I try the 550's.
My understanding is when the cable is plugged in and the nioseguard is set to off, the headphones should work passively. This is the case when I attached them to a phone but not the case when I use the 3.5mm to 6.35mm adapter to connect to the pre-amp (I've tried various adapters). The headphones have sufficient charge but I think this shouldn't be an issue once they are plugged in?
Any advice would be great as I know these are a quality pair of headphones!
I know this is a super-delayed response... but I've run into a similar issue and have figured out the causes of my problem which may help you make some tough choices. Be aware that:
1) the PXC 550's are 'Active' unless the battery is fully discharged. You can disable sound modes, noise canceling, and Bluetooth, but the circuitry is still in control unless the battery is discharged, even when using the wired connection.
2) When 'Active' the impedance of these is 490 Ohms... which is quite high resulting in needing to crank up the volume on your headphone amplifier. the problem is...
3) When 'Active' there is an IFE (In Flight Entertainment) loudness protection feature in place designed to reduce the volume level of incoming signals. This is to prevent the captain's and stewards' PA announcements from splitting eardrums in the middle of listening to music. Unfortunately, the volume is only reduced and is NOT ever raised again. So any time the signal goes above some magic level, the headsets effectively turn themselves down. The only way to reset the volume is to reset the headsets (taking them off and twist-clicking them off and then back on again).
This information is found in the manual (page 39 of the English variant under the subheading 'Using the Audio Cable')
So, in my situation... As I began to turn up the volume to a 'fun' level, the headphones started turning themselves down. Eventually, my DAC/AMP volume was maxed out by I was still only getting mediocre volume and the disconnect between the apparent volume control and the sound I heard was driving me crazy. I suspect you're encountering similar anomalous experiences.
Potential solution: Turn the headset 'On' and allow the battery to discharge fully. This should have two immediate effects: 1) Reducing the impedance to a much more driveable 46 Ohms (Passive loading) and 2) killing the IFE/PA protection features which attenuate the volume. In theory, these should result in the PXC 550 behaving like a much more 'normal' passive headphone.