Hello!
I have the XBA line earphone as well and if you are using the stock eartips, it shouldn't be the fitting problem but in case it does, here's how I instert them: [Sorry, I do not have my portable camera with me to take you any picture rather than typing]
1) Ensure that, your eartips are securely fitted in the nozzle.
2) Since it's a J-cord, I first hook it around my neck then wear the left earpiece to my left ear and right earpiece to my right ear.
3) When you are inserting it into your ear, make sure you pull your ear up a bit then insert to ensure a good seal and noise leakage. [Pretty sure you do this].
4) Ensure the headphone jack are fully inserted into your Walkman
All the above 3 steps, either one of it can be the possibility for differences in volume.
My question is, have you ever remove the eartips? If so, refer to Step 1 above. If not, try removing it, and replace it with different size of eartips to see if there is still any volume differences.
Lol...I too love my XBA as well XD
Hope this helps!
Billson