Having worked in a past life for a high end audio retailer, I can attest that their are many things in a PC that will add noise and other colorations to music reproduced through a PC.
Going back to my audio modification days, what you need to do is “pooge” your PC.
I presume since you are working with an audio shop that you can get appropriate assistance from the service technicians for this project. Both electronics design and rework skills will be required.
Start by replacing the switch mode power supply in the PC with with linear supplies. They will not fit in the case, and you will need at least 2 different linear supplies to cover all the voltages and currents needed to run a PC.
If you end up mounting the power supplies in an external case, be sure to install bypass capacitors at the motherboard power connector and at the drive power connectors.
Use high quality supplies like those from Lambda or Accopian.
While you are wiring up the power supplies, replace the connectors with gold plated pins and as large a gauge wire as will properly crimp in the pins.
Next, either switch to liquid cooling, or externally mounted AC fans. AC fan motors do not have the commutation noise that a DC fan has.
Find some old copies of “Audio Amateur” magazine and read through the “pooge” articles. Then, go through the sound card and apply the “pooge” principles to the audio circuit capacitors and output amplifiers.
In fact, if you are using a “standard” sound card, you might want to start by switching to a dedicated audio D/A converter card.
The next step would be to “pooge” the power supply capacitors on the motherboard.
You might also experiment with Faraday shields around the D/A card or the CPU and clock circuits.
Don’t neglect clock jitter. It will take some design work. But, replacing the software controlled CPU and PCI bus clocks with stable crystal controlled oscillators might be where you will get the biggest gains. Especially if you are streaming digital audio data.
One place you might contact for advice or assistance is Audio Advisor in Grand Rapids Michigan. They used to perform modifications to high end audio equipment. And still sell tube and other high end gear.
Experiment and enjoy.
Chas