Thank-you all for your insights, very much appreciated. A few questions to address:
Budget is flexible but should be in line with the system as a whole. I'm in New Zealand and the system as a whole is currently sitting around NZD 20k.
In terms of cabling / connections:
The PC is connected to the Vega via a Supra USB cable.
The streamer is connected via a Kimber Illumin digital coax cable.
The interconnect from DAC to AMP is an Audioquest King Cobra. I tried the AQ Water, but it just sounded odd in terms of imaging so went back to the King Cobra. Now wondering if the Cardas Pure Copper could improve on this.
Speaker Cables are Transparent Audio Laboratory Musicwave Plus 10AWG. Currently bi-wired.
The mini sounds clearer to me, but has more of the issue.
The microphone suggestion is something I have been thinking for a while. The room is a known big issue, large, tall ceilings, prone to echoes, odd shape, just about everything you don't want. Measuring would be very revealing. There is a definite base bump at about double the listening distance, and this does smooth things out somewhat. I do wonder whether any changes made to accommodate the bad room could be detrimental to the system in a good room.
The Vega filters, interesting, I will try this again. When this was first added to the system I had a play, but couldn't hear any difference. That said, several upgrades have been made since that could reveal this. Each time I feel like the sound is technically better, but somehow less engaging.
EQ, I've not explored options for this. Wouldn't know where to start. I'd prefer to find a permanent balance if possible.
And a couple of comments that strongly indicate the Chord sound won't be helping things here. The 2Qute is currently sitting at NZD720 with a linear power supply. Tempted to secure it for a second system in the future. The problem with New Zealand is the lack of ability to try before you buy. You either buy and sell again if it isn't a good fit or find someone willing to lend you their gear...
Network Filter could be an option for the PC, but the mini is connected via WiFi. Auralic recommend this approach.
Output impedance is an interesting question. I don't have answers, but an interesting question! I can only find information on the pre-amp section:
"The first tube boosts the input voltage by only 7dB (2.24 times) while the other two tubes, connected in parallel in repeater mode, are matched to a transistor termination. In such a set-up, the output impedance is radically reduced, allowing the MOSFETS to do their job with minimal distortion."
