I am not surprised that you are happy with your system!
Your KEFs are brilliant speakers, in part because they try to emulate a point source of sound by having the tweeter and midrange concentric. This means they throw a very wide soundstage, are much less critical of positioning than ’conventional’ dynamic speakers, and do not exhibit the severe comb-filtering that others correct with room treatments. For those who are new to these speakers, KEF has a truly excellent 40-page white paper which explores their design decisions.
There are a couple of upgrade options for the speakers. The obvious one is to replace them with the latest meta incarnation. In principle, meta is a small plastic tuned absorber disk that sits at the back of the tweeter. A myriad other changes were needed to accommodate the meta disk. While meta technology is very successful in KEFs smaller speakers, I think the Reference Series is so good that the benefits from meta are marginal.
The other option I’d suggest will be counter intuitive to most people on this site. It is to consider moving to the smaller Reference 1 speakers. All the Reference Series use the same concentric tweeter / midrange. The difference is in the number of bass drivers (which are all identical as well!). The Reference 1 has a single bass driver, the Reference 3 has two in a d’Apolito array, and the Reference 5 has four. The more bass drivers, the more likelihood there is for comb filtering, where the time delay in the different signal paths causes cancellation and reinforcement.
I have used Quad electrostatic speakers for about four decades so I am used to accurate sound. I recently added a pair of Reference 1 speakers (not meta) and I prefer them. They produce a staggering amount of quality volume and credible detail.
Your DAC has separate paths for Direct Stream Digital so one upgrade might be to buy a Super Audio Compact Disk transport, which I would recommend if you play much classical music.
