For completeness I have to report that I am now back to Raspberry Pi3 from Sparky as I was having issues that initially I thought were with HQP but turned out to be USB output. A kernel update went 99% of the way to resolving this for me but the 1% is enough to make me go back. (I'm not sure if this is just an issue with dietpi and allo sparky.
if you search "sparky kernel USB HQP" a thread in the Roon forum should come out near or at the top with further details.
I'm sorry to hear about your difficulties with the Roon implementation on the Sparky.
I've been using the Sparky and DietPi for about a week but with Squeezelite as the player. I have also encountered something rather strange in the installation.
The autostart at boot for Squeezelite didn't work. I could see that it did start but it appeared that something caused it to be killed later in the boot process. If I restarted the Squeezelite service manually there were no glitches and the player worked entirely reliably. I tried a number of things to try and resolve the boot issue but more or less became reconciled to manually starting it after each power off.
A few days later, I became a bit irritated by the appearance of the USB cable - it was a rather stiff Audioquest model and needed a half twist between the Sparky and MUTEC and it was sticking up in the air. I found the cable ran better if I used the USB3 socket in the Sparky rather than one of the USB2 sockets that I had been using. And.....low and behold..... the problem at boot disappeared! Just how weird is that! I don't think the DietPi installations are quite as fool-proof as Max2Play or PiCorePlayer, for example.
I'm finding the sound through the MUTEC and NDAC to be really excellent. For now at least, it's my preferred player.
I have also been playing with one of the new HiFiBerry DIGI+ PRO boards. I noticed that there wasn't much difference between the DIGI direct to the NDAC and through the MUTEC, so HiFiBerry have probably done a good job with the new clocks. Through the MUTEC, the PRO and non-PRO versions sound pretty similar to me.
For me, the Sparky just beats the DIGI+'s in having a little more life and clarity. A bit more "sparkle" I might say!