It's very disappointing there's no G91 upgrade path (or support for that matter when it's faulty as I discovered). So, what to do ? - I just listen to music in stereo.

For me the answer was a DEQX HDP3 (new model) into a valve preamp.
This gives me a true high-end sound. The "Meridian" sound colouring has gone. The previous G91 DAC-preamp sound is just weedy in comparison. So it was a great upgrade. Comparing DACs - HDP3 is way smoother and realistic.

Interestingly my modded Pioneer 575A playing CDs seems to now match the G91, while SACDs are superior on the Pioneer. It's great to see technology moving on and possibilities for a much lower-cost top-end systems.

Next upgrade is to trade the G91 for a true DSD player. There's no doubt in my mind that PCM can provide huge resolution (eg. some of the latest Bluray 24/192 recordings) but it throws away mid-range harmonics (like DVD-Audio). Maybe that will change one day but it does seem a PCM technology limitation that Blu-Ray persists in retaining.