Power Conditioners

high-amp, I do not use any power conditioners and do not feel the need to spend money on a questionable improvement. My system is dead quiet even when turned up to ridiculous volumes. I do have an unusual situation as my house has it's very own power transformer and I do have a whole house surge protector. 
Although there may be conditioners that are effective my sense is there is a lot of scamming going on in this area as there is in cables. It is just the way humans are.

Expectation bias leads a lot of people to think they hear an improvement when in reality there is no difference. The question is, is a perceived improvement as valuable as a real one. My own attitude is absolutely not. 
I tend to ignore subjective evaluations and look for objective evidence such a lower signal to noise ratio. Obviously object evidence does not tell the whole story especially with amps and speakers. These evaluations you have to make on your own.

Vinylshadow, the power supplies of your amplifiers are power conditioners. If you feel the need to add more power conditioning they are terrible amplifiers and you should get ones that have decent power supplies and make sure they have balanced inputs that you actually use while you are at it. Balanced interconnects running higher voltages is the best way to lower noise.

Vinylshadow, use your Gray on the line level equipment and plug the amps directly into an outlet. Get a whole house surge protector.

Vinylshadow, you could plug an arch welder into a 30 amp line! You have to remember that the impedance of all these devices as as much to do with current draw as anything. They are not low impedance loudspeakers. An amplifier can put 3 amps into the loudspeakers while only drawing 1/2 amp from the wall. It is converting voltage to current. 12 amps is more than enough to run all that you mention. You are probably only putting a watt or two into the surround speakers. People have this crazy tendency to over power their systems. The power supplies represent the choke point and most line level equipment and some power amplifiers are tightly regulated. Putting fatter wire in the wall is not going to make them perform better. Amplifiers that might overwhelm a 15 amp circuit are generally run 220 volt like the big Boulder. Also frequently forgotten is that it all goes back to the same panel so from a noise standpoint it is all being hooked together. Hopefully your Gray will isolate your more sensitive line level equipment. My own system is as quiet as a mouse. It is dead silent even with the volume cranked to the max and I use absolutely nothing. When I turn on my projector when the lamp fires up it will make the system stumble I put a transformer on it and it still stumbled. I put a power regenerator on it and it still stumbled so I sold it all, takes up too much space. If you have a source of noise in the house say like the refrigerator see if the Gray protects your system from it. 
Use Balanced interconnects with your amps if you can. Tie wrap your cables neatly into two bundles one power, one line level. If a power line has to cross a line level interconnect do it at 90 degrees for less induction. Nothing bothers me more than wire thrown around like spaghetti. 
The Siemans is a fine surge protector. I'm not sure which one I have in there. The tech installed it when the generator was installed. Many of them are sacrificial so it is a good idea to keep a spare around. I'm not sure about the Siemans. The manual will let you know. You'll have to turn off the mains when you install it so have plenty of battery lighting ready!

I forgot to mention that you have circuit breakers which are going to limit the amount of current you can draw from any one line and modern breakers are quite sensitive. You will know if you are overloading a line.