High resolution audio on a PC questions

You can always keep the DX3+ for a spare for when your more expensive stuff breaks.

Also, I have an Onkyo TX-8255 audio receiver (it’s completely analog). Would I hear an improvement if I ran the signal from my PC’s headphone jack through the receiver? (it doesn’t have a line out). Sorry if these are stupid questions.

Probably not simply because the signal out of the headphone jack is analog processed by much less quality internal dac of PC. You need to use the USB to get the digital signal out connect to external dac as already mentioned then receiver/amp. The difference in SQ could be night and day.

Here's a simple explanation:

DAC is the acronym for "Digital to Analog Converter"  -- in short, it takes the digital signal from the CD or music file and converts it into an analog signal that can be amplified and played through your speakers -- in other words, an analog signal just like you'd get from a radio tuner, phonograph or tape deck.  

Now ask yourself, do all tuners, phono setups and tape decks sound exactly the same? To most who are serious about their hi-fi setup, the answer is no. That's why you have forums like this where people discuss things in their search for the gear that best suits their preferences within their budget. 

This is a really deep rabbit hole.

First, I believe that @yage is correct. Macs don't play nice with high bit rate music. I think there might be a workaround, but I am not sure. Windows and Linux will pass high res on with no issue depending on how you implement it.

It is difficult to get really good signal out of any computer.

My suggestion is to buy or build a dedicated server, you can get an i5 NUC for a reasonable price. I suggest running Debian Linux (no I am not a Linux fanboy). it is both easy to use and easy to install, it will not feel alien to you at all. You can install Debian Server and take the option for a desktop interface so you don't have to screw around with the command window. This is a huge help when dealing with setting up the networking (which is easy on the desktop).

Better is to run Daphile (which is about as good as it gets), but Daphile is headless and must be set up via the command window from a remote computer. The Daphile instructions make it pretty easy to do.

Alternately get a Windows PC, and run audiophile optimizer and fidelizer (this is a pretty good set up, not as good as Debian).

You don't need anything powerful, i5 is more than enough.

There are a ton of options for getting the digital signal out, generally speaking Ethernet will be the best, wifi might be the next best, and USB will be the worst unless you spend a bunch of money on a fancy USB board.

A good option is the USBridge from Allo which will convert Ethernet to USB. This bridge and others like it use a Raspberry Pi, and contain the software for the user interface which you operate from a phone or tablet. I prefer Volumio.

Find a decent USB DAC and you are good to go.

You can also get a streaming DAC, many of the lower priced entry level DACs have streaming built in and will be able to fetch your music off your server. 

If you have an old Mini (never use a laptop), or Windows machine laying around this will be relatively inexpensive and such a massive upgrade you will wonder why you hesitated about upgrading in the first place.

I started down this rabbit hole running Daphile on an old PC, and an Allo DigiOne connected to the digital input on my old NAD AVR. One thing led to another and after years of upgrading I am using an i9 SonicTransporter server running HQPlayer and Roon, a Sonore Optical Rendu and a Holo May KTE DAC. So beware, it can get really expensive.