I am starting to wrap up my own power tube testing on my LD MkII. It took me some time since the EF91 GEC tube was so clear sounding with the power gold grid 6N6P tube. I had to readjust the setting on my Behringer SRC2496 DAC, which is the master DAC. It is now easy to hear the difference between 16, 20 and 24 bit internal processing option. I also can reliably tell the difference between line and internal clocking options. I also had to readjust the internal sampling rate on the SRC2496 for best clarity and it was not the same setting as before. The SRC2496 then feeds my slave DAC which is my brand new RDL HR DAC1 DAC by way of an AES connection between the two DACS. Both DACS uses XLR balanced outputs for which I use a conversion PylePro cable that switches the XLR balanced outputs to RCA outputs to my two Little Dot amps. My slave RDL HR DAC1 DAC drives my Little Dot MKII.
The gold grid 6N6P power tube is punchier and more dynamic sounding than my standard Russian 6N6P tube. Both have good tonal range. The standard OTK 5 Russian 6N6P tube is a more relaxed and just a bit softer sounding tube than the gold grid Russian 6N6P. Both tubes are good, but the gold grid 6N6P tubes are hard to find. Both power tubes have great synergy with the EF91 CV4014 driver tube.
On the other hand, the Sovtek 6H30 power tube is awful sounding. It was very thin sounding and had very little bass compared to the previous two tubes that I mentioned above. The EF91 GEC tube, which is just a bit brighter from neutral, just makes matters worse with the Sovtek 6H30. The synergy was not there.
I did not try the Electro-Harmonix 6H30Pi Gold tube but depending on what DoA finds on his power tube reviews I may revise my decision.
I ran into another surprise. My Denon AH-D2000 headphones, which I was thinking of selling because of the burry bass, huge recessed midrange, and peaky treble, were transformed by the GEC EF91 and the gold 6N6P power tube combination. The midrange came back and the bass sound much better. The sound stage on the Denon is now huge, even better than the Sennheiser HD 598. However, the Denon headphone at 32 ohms is showing a bit of a strain on my LD MKII amp and I am in process of doing an experiment to see if I can make the Denon headphones to work even better.