Denon, i'd agree the preamp section in the CD-5s is good enough to knock off the Ref 3. In regard to your Krell 302, the imput impedance is set very high on xlr & should be benign to even the weakest tube preamps.
In regard to the metallic "ping" which you hear when you press play on your Ayon, this comes from the 6H30 tube filament and is very normal. Ayon do not use any negative feedback or compensators to erase tube noise or hum as most manufactures are doing. Boulder are the only company which properly understand negative feedback, but they are an exception. Most other applications of GNF have a negative impact on the sound.
Regarding the noise issue you're experiencing on "loudly" recorded cd's. Firstly, xlr cables help lower the noise floor through common mode noise rejection & better grounding, but also roughly double line level gain. The CD-5s already has high gain, and due to your Krell's high input impedance no doubt your over-juicing your amp which is causing the distortion. As a starting point, i'd recommend leaving the gain toggle set to 'low'. Remember the clipping point doesn't change with balanced cables, only the position on the volume control where that occurs. If the above doesn't solve your issue, i'd switch back to single ended cables which are apparently lowering the line level gain to an acceptable level.
Concerning the 6h30DR tubes, yes they are much superior to the factory EB's and provide a substantial improvement. But given the diminishing stocks of these tubes, I wouldn't have (personally) posted my source for the tubes, unless of course I didn't mind stock levels running down to ever looser matches & prices correspondingly increasing. My 2 cents.