ok, let me backtrack a bit and first ask what you mean when you say that "the single ended output seems wired balanced"? it seems like a contradictory statement. or are you just saying that it "sounds" balanced?
as for those 6db, balanced output is typically 6db louder. i'm not sure if that's necessarily true for all players, but it certainly holds for the PD2. if you compare output volumes from the 3.5mm output and the 2.5mm output at the same numerical volume setting, you'll find that the 2.5mm output is notably louder. that difference persists even if i connect to the 3.5mm output with a balanced cable. this to me strongly suggests that the 3.5mm output is a regular single-ended (non-balanced) output. if you think otherwise then i'm curious to learn why.
@counterclockwise,
To answer to your questions, I stated
'the single ended output seems wired balanced' as
ALL 3.5 mm Balanced I plug into the PD2 work with no issues whatsoever & since I'm not about to open my unit up to check, this is more than likely the case, hence the word
'seems'.
As more often than not, as standard, nearly all other 3.5 mm connections do not work with 3.5 mm Balanced as most aren't wired for it.
I have used a lot of DAPs & head amps with balanced outputs & more often than not, especially on head amps, I notice a decrease in volume rather than increase on balanced connections & on any balanced DAPs, the sound levels are either the same or if they have independant volume control, either different or the same, depending on what was set before.
Honestly, I find no difference in sound level or volume output from either the 3.5 mm or 2.5 mm & have tested this many times with different head gear so not sure why you think there is a difference in volume or level.
I also listen at lower volumes (Average 40 dB on PD 2 / Average 20 dB on PD) than most people so I am curious how many dB you usually listen at.
So short of using a dedicated dB meter, I stick to whatever sound level I am comfortable with.
As a final note, I am not a fan for 2.5 mm sockets or plugs.