That is a really old book. Peavey is using NJM4560 dual in line package op amps for their mixers. I redid a Herald electronics mixer that originally had NJM4558D's with ST33078's. 725's were two decades ago or more. The 5532 IC was available then, and still is used a lot today for low noise and low price.
Microphones come in two classes, dynamic and condenser. Dynamic mikes tend to be about 10kohm impedance. Phantom Powered (48 vdc) condensor mikes have an output impedance in the tens of ohms. This allows them to drive long cables <33 m without losing the high frequencies. Better music uses condensor mikes, DJ's use dynamic mikes like the Shure SM58 or even the SM48.
Line level inputs tend to be for devices putting out 1.6 VAC full scale, or 2 VAC in the case of some more modern devices. Power amplifiers tend to have the same specification as the input level for full power, although many PA amps have an input attenuator. This means the line level stage must have gain from about 5 to 1:1.
There are many mixer schematics on this forum that are better than these old ones. Check this thread.
http://www.diyaudio.com/forums/analog-line-level/167604-basic-mixer-design-need-input.html Ron Elliot's westhost site is one of the standards of the internet of internet schematic projects, I believe one of the schematics is shown in pdf (adobe) form on the above thread.
Anyway, gain of an op amp is basically Rf/Rin where the Rf is the feedback resistor from the output to the minus input of the op amp. I modified a commercial mixer kit with hissy hummy sound to sound very decent, in this thread:
http://www.diyaudio.com/forums/anal...improving-disco-mixer-mid-fi-performance.html There is an idea there for a power supply for 5 to 6 op amps using a surplus wall transformer, two 5W zener diodes, some resistors and caps. A headphone amp included in the mixer would require an actual voltage regulator circuit or bypass power transistor on the zener diode.