Audio line level conversion from consumer to unbalanced (presumably) pro 1/4" jack of unknown sensitivity and impedance. 400W 1980s PA amp
So it sounds like you just need a stereo +15dBV pre amp.
If you search 'Stereo LINE PRE-Amplifier, 15DB, 12VDC' you find a pre built box that will do this for you.
or... you can go the audiophile route where you can spend +$10k on just the pre-amp... but they promise you have angels dancing in your head ;-)
In regards to your circuit,
you want a gain of (10^(15/20)) = 5.6 times.


U1A has a gain of (19.6/10)= 1.96 
U2A is just a buffer 
U3A is a unity gain inverter and the differential output multiplies
the gain x2,    thus about x4, still not quite 5.6


If you want to roll your own, increase R2 to 56k, change U2A to non inverting and tap your output from U2A via a 100R resistor. U2B not required. Make sure you use an audio grade op-amp with a gain bandwidth of 500kHz.