This is typically the problem with homes or apartments that only have one outlet in the room. I would first suggest getting quotes from qualified electricians to run at least two dedicated circuits (separate hot, neutral and grounds) to the panel. In my opinion, having everything plugged into one outlet (and turned on), will cause voltage and current problems that may be audible.
Raised foundations homes are easier to run new circuits than concrete foundation homes. Its easier to run circuits under the house than to run it through ceilings, attics, walls and floors.
A decent power conditioner is a really good idea for the low level electronics, such as pre-amp, cd player, tuner, turn table, DAC, HT Receiver, etc. and have that power conditioner plugged into its own dedicated circuit. I suggest plugging the amps into their own dedicated circuit.
You can get away with two dedicated circuits that way. Better if the home theater stuff has its own, but, its not the end of the world.
Leave all unused equipment off and you won't have any noise or "digital" noise issues.
I use a Transparent Audio Power Isolator 8 unit that I compared with four other power conditioners and heard a definite audible difference for the better using the Transparent unit. I literally did A,B,C & D comparisons and the Transparent unit was definitely better.
Costs, or if one lives in an apartment, getting permission from the owner/manager to run dedicated lines are typically the issues in the dedicated line discussion. If you absolutely cannot run dedicated lines, then a good power conditioner that isolates the digital circuitry from the rest is the way to go and again, turn off everything that you don't need on at that time.
Can't have digital noise corrupting the rest of the audio system if the digital circuitry isn't turned on. Same for HT vs audio.
In my case, with the dedicated lines (I ran four). The noise floor dropped to a point where its gone. Also, using the power conditioner as I described above with my three amps (two Audio Research REF 250 mono amps and one Mark Levinson 23.5 amp) plugged into their own dedicated circuit, I absolutely have no noise issues, no ground loops, hum, and with all my electronics on at the same time (including DAC and CD transport), I have no digital noise on my system at all.
But, like I said earlier, if you can't run a dedicated line, a good power conditioner (they are all not the same) and turning off all non-essential electronics will help dramatically.
enjoy