Inexpensive but good sounding power supply (and hard drive) for PC audio?

Oh, I have tried working with Fidelizer and it did weird things to the sound. I think using the stock configuration in Windows is best, but there are a lot of people who think Fidelizer does good things.

I have also heard that trying these expensive high end OCXO clocked routers can also do weird things to the sound.

I have had some considerable experience with sound from a "PC".  As far as computer power supply is concerned, linear power is better.  The HDPLEX options are recommended, but they are external units and will require some modifications on the power wiring to your computer components. 

If you are looking at normal PC power supplies, I would recommend getting the largest you can afford.  They are all switching power supplies, so the larger sizes will have larger capacitors as a power bank.  This equates to smoother current/power.    The Corsair AX1600i measured to have the cleanest power output.  However, the cables that Corsair supplies are heat-shrink and techflex coated.  These are extremely thick and stiff cables.  If you are trying to route these behind the motherboard in small spaces, the Corsair cables will NOT work beause you just cannot bend/flex the cables enough to fit through tight areas.  The Seasonic PRIME TX-1600 is almost as good and they have much better cables for routing/bending/flexing. 

For transfer of signal to your outboard DDC or DAC, I have found that USB quality is really dependent on how good the USB-to-i2s circuits are on the DAC.  I, personally, don't think the USB is the best mechanism, but this is a controversial subject.  I have a friend who owns a $25,000 Pink Faun music server that includes both a Pink Faun USB card and a Pink Faun AES/EBU card (both with the highest level OCXO clocks).  When using the USB output, it gave the highest resolution for midrange and high frequencies (air), but it just did not sound as natural and full when compared to the AES/EBU output.  This was going to a $16,000 DAC (I can't remember which one it was).  The AES/EBU output had more weight, bass strenght, and overall fullness - plus it sounded more natural and real.

On my own system, I am running a stock level version of the Pink Faun AES/EBU card with a Farad Super3 power supply.  This is just outstanding.  The Pink Faun s/PDIF card with AES/EBU and BNC outputs would be my recommendation if you have that input on your DACs.  That being said, if you are running a Linux operating system with Intel CPU, this card does have problems.  Running Windows on Intel should be fine (it's a weird chipset comparability issue).

"Euphony OS for Windows" does not actually run in Windows.  They just have a "Euphony Downloader for Windows" which can be used to download and written to a USB device as a "bootable device".

Euphony is Linux operating system that is similar in ways to "Audiophile Linux" or "Arch Linux".  My understanding is that Euphony has done a lot of the "common tuning" which creates a result that is great in most scenarios.  Hardcore Linux users can take ArchLinux and spend hours/days/months tuning the O/S configuration.  In the end, they may achieve better than Euphony (or worse depending).

In terms of the PSU itself, I don’t think that will impact sound quality. Because it’s a computer, not an audio component.

All good DACs will pre-filter the USB input from any noise over USB, so that’s not a concern.

I disagree with this based on personal experience.  The power supply will definitely affect a digital source.  My own experience is with a Pink Faun S/PDIF PCI card mounted in a computer.  Using the computer's power supply, the sound was nice, but I can definitely hear harshness and noise/distortion in the sound.  When I connected an external Farad Super3 linear power supply to the Pink Faun card, the sound really cleaned up and became more solid and more powerful.

Even in this case, the quality of the computer switching power supply will affect the digital signals being passed to the Pink Faun S/PDIF card (even though the Pink Faun itself is powered by an external linear power supply).

Actually the opposite is often the case. Nothing could be more inefficient than a Class A power supply.

@cleeds  - I assume you are meaning this as a joke.  99% of all computer power supplies are switching power supplies (the most efficient type).  However, there are differences in performance between manufacturer/model of power supply.  Even HDPlex, which is a computer linear power supply, is not going to be a "Class A" power supply like the shunt regulated PS circuits that you find on high end preamp/sources.