-10dB/decade noise floor slope is interesting. What is the reason?
Computers have switching supplies that often put out lots of powerline and radiated hash especially under load. Some brands of computer power supplies are better than others in this regard. I don't know for sure, but I would guess that a top brand like Seasonic would introduce much less noise than bargain brands.
You can do a rough check for noise that your PC is putting out by placing an a portable AM radio, tuned between stations, near the PC's power cord. You will hear some of what the power supply is putting out onto the power line. This could be part of your problem.
Corsair should be OK, yeah.
If the noise is coming over the power line, you might try plugging the computer into an isolation transformer - this one is good for 450 watts, check to see what your PC is drawing off the line when it is running, this unit may be large enough or you may need a bigger one https://www.amazon.com/dp/B00R5CDG8O/ref=cm_sw_em_r_mt_dp_E438D0NPNSZC7A3PH6G9
If you are hearing noises when the GPU is under load, this tells me that noise from the GPU drawing bursts of power off the 12 volt power bus in the PC as it is doing it's thing is generating noise that is getting into your speaker's amps, this could be through their audio connection, through their power connection- or through the air via RFI. Powerful GPUs draw huge currents- very short bursts of hundreds of amps at 12 volts.
The problem is that there is a ground loop due to different ground potentials between components in your system. Once this happens all sorts of noise from many different souces can get picked up. You can never eliminate all the noise sources but you can break the ground loop. You have solved the problem by breaking the ground loop with a transformer which is one of the best solutions...The other solution is balanced connections. Enjoy your quiet system.My GPU and CPU can separately draw 450/400W peak at stock voltages. I would likely need to get a 1800W transformer and I'd rather not spend $600+ when the Jensen seems to work well enough. I don't know how the noise is transferring, whether through the connection or through the air as the receiver itself is located very close to the PSU as are any number of electronic devices. I know that the speaker itself isn't audibly affected by proximity to the PSU when connected to the transformer. At some later date I'll see if distancing the receiver has any effect.