Star grounding is a grounding technique that connects all the ground wires to a single point and than connects that point to the ground. The resulting topology looks like a star and hence the name star grounding.
Ground loop hum is caused by the relative voltage differences at the ground levels between different components. So if you plug you power amp into one outlet and plug your preamp to a different outlet, it is possible that the ground wires in the two outlets will have different resistances and therefore a voltage difference is developed between the two amps. When the outlets are new, the difference in resistance may be small and you may not notice much noise. But as time goes by, oxidation will develop on the contact points of the outlet, the bare wires, and the circuit breakers; and the noise will get worst.
If you plug both amps to the same outlet, the relative resistance is measured only between the two power cables of the two amps which is much less than going through the outlet, the wires in the wall, and the circuit breaker, and back.
I am not sure what you mean by "two 20 amp lines with a common ground back to the panel". But if I were you, I will plug the Juice Bar to one 20 amp line and than plug everything to the Juice Bar.
I hope this help.
