Cue Audio Issues

Hello fellas, just wanted to check if anybody has been having problems with their audio output via the headphones on the front of the S4. At first I thought I was having issues with the actual headphone connection, and after browsing some fora thought it could be the "tighten the nut" issue as I am using MDR-V6's with a threaded adaptor. I used several adaptors and cans, as well as tightening the nut with no positive effect, so I started going through setting combinations and found this oddball:

1 - I was not getting a full audio spectrum from my monitor output, it was heavy on the mids and highs but lacked bass. Also, my 'CUE VOL' knob would reach max output at around 12 o'clock and any further movement seemed useless. My settings at this point looked like this:



2 - Hitting the 'MONO' button had no effect on either audio or volume:



3 - After a few fiddlings with the settings and all deck levels in zero and 1 deck in CUE I hit the jackpot. I disconnected one of the monitor outputs and suddenly, VOILA, full audio spectrum and a functional volume knob all the way through:



4 - Curiously enough, it works like this too:



5 - In points 4 & 5 hitting the 'MONO' button has no effect (as one would expect, there being a single monitor channel to output). Also, when I reconnect both channels I go back to the "lacking" sound and volume drops considerably...

If you are having such issues or know if this should be so and I am completely being stupid let me know...