After using the amp for around 20 hours, I re-measured the tube bias. I found that it drifted to sub 10V. So in this whole process, I adjusted the B+ to 26.8V and thus biased each tube at 13.4V. This would result in around 12.6V on the heaters with the 10R heater resistor. I found that the sound improved and tighten up a lot. Over the course of the next 100 hours or so, adjusting for bias every 20 or so hours. I also took the chance to change the buffer bias to 110 mV. Regardless of these changes, the amp remains deadly silent with no hiss, hum or noise through the HD650. With my neighbourhood being very quite even during the day, it makes for a beautifully silent background.
It would be interesting that after the first 100 or so hours to play around with the tube bias and the buffer bias to see its effect on the quality of sound. It would appear from previous experiments that running the tube hotter and closer to its limits tends to produce a bigger and more dynamic sound. This amp would be an excellent platform to test such differences.
One of the delights of headphone audio is that you can really start to hear the music in a way that you cannot with speakers. Some faint spoken lyrics or background vocals becomes crystal clear. You can hear subtle noise in the background, like the creaking of piano pedals, or the pianist moving in the seat. Such minor details, though not affecting the overall enjoyment and appreciation of the music, tends to adds a new dimension to provide a much memorable experience.