How to get into high end digital? (Feeding a DAC)

Fiber optic provides complete isolation from the noise in the power supply of the sending unit.  Coax does not.  Galvanic and isolation transformers still pass noise through the ground.  Wireworld makes nice fiber optic cables made from glass.  Although I also have others from Amazon.  
For a transport I have a Rega, Woo Audio, and Marantz.  The Rega sounds a bit thin compared to the other 2.

For streaming I use Qobuz for my IPad into an Upsampling DAC.

I’m a tried a d true AQ guy. I do use two top end Wireworld for Ethernet though. 

Forget all that stuff.  Pitch it all out and get a set of the Dutch & Dutch 8c.    Not only are these loudspeakers amplified, they have built-in DAC and DSP EQ.  All you need are them and a digital source like a laptop, an iPad or even just your cell phone with Roon.

When you set them up, you tune them for your room and you're done.   They have some of the best imaging I've ever heard, at any price.  Truly uncanny.

Now there's no screwing around with matching components, cables (you get to eliminate speaker cables!), changing out tubes...    One stop shopping for the very, VERY best audio you'll ever hear.

Back in the day, is when I bought my Ayre QB9 and the emphasis was in how to make digital sound analogue. The technology has changed significantly since then. I was lucky in that Ayre has updated that DAC to the present. Maybe close to eight years ago I bought the Uptone Audio ISO Regen with linear power supply and found that it made a very noticeable difference. Today on my Ayre QB9 Twenty, the difference is minimal by comparison.

The criteria for choosing a DAC today is so much more complex, i.e. detail, soundstage, noise floor, jitter, etc… This in part is due to the DAC chip but more so, how it’s implemented. And because of this, I’d still recommend trying to visit brick and mortar stores to hear as many DAC’s and transports as possible.