I know the Quedlix 5K has great balance control (and independent L-R 10 band PEQ). But that's a mobile device but you might be able to hook it up to a PC with a USB cable.

On the much more expensive end (~$900) is the RME ADI-2 DAC/amp. That has extensive L/R balance capabilities and many more DSP features.

And like /u/Zilfallion says, checkout Peace EQ (which is a GUI interface of Equalizer APO). You can adjust Left and Right to your hearts content without having to buy one of the devices I listed above. Example image