I am running an HDMI cable from my Mac Mini into the Denon AVR. Do I need a Streamer, as I can source all the streaming services on the Mac. Tidal & Qobuz, and local hi-res audio is going via Roon. I can also feed live radio from Roon, and I realise this is lossy. I can also push other services such as Soundcloud, and Bandcamp through the HDMI link as well. What could I do to improve audio quality without breaking the bank?
After acquiring the K13, you could repurpose the DX1 as the DAC to feed the Denon either with a longer USB cable, or better, a ROPIEEE setup.
A Raspberry Pi Zero 2 W Basic Starter Kit is around $50 US, plus need an SD card and download the software. A little more for RPi 4 with built in ethernet. Fairly straightforward setup and Great local support from Roon Forum Latest Audio Gear Talk/RoPieee topics - Roon Labs Community . This would be your Roon endpoint for any future DAC and will pass any format your DAC can take.
Some streamer options are available for more money but read the specs cautiously as many actually just use Airplay as the transport or do not support passing DSD to USB connected DACs.
If at some later time, you feel the need for a DAC upgrade – whom are we kidding here? When you wish, feel the need or can’t resist the urge to upgrade your DAC further, just swap it out with the DX1.
Welcome to the slippery slope. It started with a wire.
For audio quality, that should be about as good as it gets.
You want to get the digital signal to a DAC (Digital to Analog Converter) for conversion to analog and then amplify that analog signal - and no more.
Your most recent comment about HDMI does suggest that’s what you are doing. The Mac is acting as a digital streamer/file source and the digital signal is sent to the Denon via HDMI. Your Denon does the digital to analog and amplification internally with its own DAC and amplifier.
What you don’t want is two DACs in the chain. i.e. Digital to Analog to Digital to Analog.
I may have misread your first post. I was assuming you were going to send the digital from the Mac to the Fiio and then the analog output from that to the Denon. In this case, the Fiio would have made an analog signal for the Denon. Then most likely the Denon would have flipped the analog back to digital for processing. Finally, it would use its own DAC to send analog to the amplifier. You get multiple conversions and the audio will only ever be as good as the lesser of the two DACs.
Note: Most AVRs offer some form or “Pure Direct” mode where the AVR leaves the analog signal unmolested. This is the only way to prevent the analog back to digital scenario. It completely bypasses all the processing, room correction, sub .1 channel etc. that you have paid for. I have always found it to sound rubbish. YMMV.
If, howerver, you are running a cable from the Mac to your Fiio DAC for headphone listening and a separate digital (HDMI) cable from the Mac to the Denon (and its DAC) then all good! 
Tl;dr;
plug a digital cable/connect into the Denon AVR.
don’t put the Fiio in the signal path