Tone control altered Balance ?

Hey,
Unfortunately the parts used in tone controls are often not as good as parts used in the rest of the audio chain, even less so in the headphone amp sections.  All sorts of things could be going wrong.

e

Did not realize this through loudspeaker

check your earphones or something is going on in your amps hedphone out.

G

See an audiologist.

I don't know how the bass control is implemented in that NAD amplifier, but if the control is a dual potentiometer then left/right tracking (balance) is never going to be great. I wrote blog post on the subject here, focussed on volume controls but applicable anywhere a dual pot is used. You may need to adjust the balance after changing other settings.

Could be the recording. If the tracks or album being played had the bass instruments positioned more to one side than the other in the recording, turning up the bass could indeed make the sound appear shift in that direction.  I'd suggest you try a number of different albums by various artists and labels. If this happens only on some of them and not others, this would suggest it is the recording and not your amplifier. OTOH, if the effect is universal regardless of what is being played, then it would suggest this is either a design oddity or something out-of-kilter issue with your amp.