Operating from the 12 VDC supply they recommend, the op amps are biased at 5.67 volts. This is set by R43 and R44 which gives the base of the power supply transistor 0.55 times the input voltage after the drop at the input protection diode and this is followed by the Vbe drop of the power supply transistor, which is not a regulator - it is a capacitance multiplier that increases C26 by its hfe. The 0.55 multiplier is set by R43 and R44. This should be OK.
The 120 pF caps in conjunction with the 47K feedback resistors give a rolloff at 28.2 KHz, which is OK for value, but ceramics do not belong in audio, especially if they are used in any loud ambient where they may become microphonic. The ganged level pots at the input have a large effect on frequency response. When set for maximum gain, they cutoff the low frequencies at 33.86 Hz at the -3db point for the input stage. The C1 and C2 electrolytic capacitors and the corresponding ones in other channels should be raised in value.
The output stage for J8 and J9 use transistors to boost the op amp output when the op amp output current reaches 23.3 mA. This is set by the 30 ohm R38 and R41. At the 5.67 volt bias level, this may not be reached. Raising this resistor will allow the power output to be handed off to the transistors at a lower level. If you were to replace the op amps with some of the popular ones like the TL072, which has 192 ohms in series with the output internally, you may never get these transistors to turn on.
The worst problem at this stage is C22 and C24 which set the output bass response. Going into a zero ohm load, the bass cutoff is 341.4 Hz with this number going down as the load impedance rises. C19 and C20 are also bad with a cutoff at 159 Hz at low impedances. Raise the capacitance there for better bass.
The 50 nF ceramic should have a minor effect - it is just there to keep the ground at chassis potential for AC signals, but if it turns microphonic, this will not be good.
This unit is a good starting point for a better mixer (and there are many worse designs out there). Just do the calculations and set your values where you want them. I am not familiar with these op amps, but you should determine whether they have the input and output range to handle the signal levels.