No star grounding in this pcb.
Star ground or not doesn't enter in. At some point there should be one, and only one, circuit ground point (star grounds, when used, radiate from this single ground point).
You have two devices, a preamp and an amp, yes? And each of these devices presumably has its own circuit ground point--one and only one for each device.
These two ground points should ideally be at the same voltage (zero volts), so there's no voltage difference between them.
The way to check this is with a voltmeter. Simply put a meter lead on each circuit ground point, and see what voltage reading there is, if any.
Important: a resistance check is meaningless. This has to be a voltage check, made with both devices turned on, normal operating conditions.
Caution: electricity is dangerous and can give a potentially lethal electrical shock. Do not perform this check unless you're qualified to do so by training and experience.
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Gain of your circuit is set by R4 and R1. The formula is:
Gain = R4/R1
Pot1 doesn't control the circuit in any way. As presented, the circuit is set and non-adjustable.
Pot1 is acting as a voltage divider, as do all volume controls, which is what Pot1 really is. The entire voltage output of the circuit appears between terminals a and c of Pot1, while the wiper (terminal b) picks off a portion of that voltage--i.e. divides it.
For a gain of about 7 replace R4 with a 6.8k resistor.
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