I've been using the OPPO PM-1 with the Metrum stack - this morning - for the first time - for about 5 hours now. It's efficient enough I've concluded I don't hear anywhere near as much loss of dynamics as I do with the LCD-2 (or even the HD800) when toggling between the Aurix 0 dB and 10 dB gain settings and adjusting the volume control to tape-marked positions established by volume matching with a white noise file and SPL meter. I do hear better dynamics with the OPPO PM-1 at the 10 dB setting, but it's subtle, leading me to believe that the PM-1 is not quite efficient enough to deliver its best at the 0 dB setting - but nearly so.
10 dB Gain brings the step-up transformer into the signal path.
Marks on tape show the volume-matched positions for 10 dB (at left) and 0 dB (at right), for a measured 80 dB SPL inside the ear pads of the OPPO PM-1 playing a white noise WAV file. (Imagine those marks moved farther clockwise for the HD800 and more so for the LCD-2.)
Both the HD800 and the LCD-2 rev.1 are too
inefficient to use the 0 dB gain setting without compromising dynamics, as heard with really dynamic recordings like Beck's
Sea Change or Michael Hedges'
Aerial Boundaries. The OPPO PM-1 (PM-2), on the other hand, are efficient enough that I can only hear a slight difference in dynamics and no discernible difference in bass control or speed when toggling between 0 dB and 10 dB. This would be the same for the PM-3, also, since it's even more efficient than the PM-1. The DAC alone might be able to provide enough power for the PM-3 (through the Aurix at 0 dB.)
Interestingly, I'm hearing a little more bass energy with the step-up transformer in the signal path than without. This again, is with the Aurix driven by the Octave MkII, where a synergy of some sort yields more bass in the high impedance HD800 than with the low impedance OPPO PM-1. So, although the bass energy is not boosted as much with the OPPO PM-1 as with the HD800, at the 10 dB gain setting, there's a detectable increase in bass energy in the PM-1 with the 10 dB gain setting vs. the 0 dB setting.
Still, there's something else happening when the step-up transformer is engaged (10 dB) - more of the euphonic tonal qualities that are so lacking in the HD800, but less needed by the PM-1, and even less so by the LCD-2 - for my tastes. Without the step-up transformer, the PM-1 is able to hear the DAC with little more than the attenuation provided by the volume control and some impedance matching, but if you want an even less "sterile" sound - of which the Octave MkII can hardly be accused, using the step-up transformer of the Aurix will deliver it - I am absolutely certain of this - tube-like goodness resides in the step-up transformer. I couldn't determine this using my less efficient headphones because they "need" the extra power of the transformer, so 0 dB really isn't an option for them - if you want to maximize their performance overall.
In an attempt to quantify this "euphonic" contribution made by the Aurix transformer at 10 dB of gain, I'll say that I'm still convinced the Octave MkII makes a much greater contribution to euphonics, when used with other amps, than does the Aurix when used with other DACs, but the best increase in bass energy seems to come from pairing the Octave MkII with the Aurix while using the transformer, and they are more effective in this regard with high impedance loads than with low impedance.
I hope I'm right about all of this stuff. When I start making really specific "observations" like this I get nervous, because I do feel a burden to be accurate for the sake of others who might act on what I write. Still, I'm saying it as I hear it, so hopefully my findings can be found reliable.
With that disclaimer, I'll stick my neck out a little farther by adding that the treble energy is rolled off a bit with the transformer in the signal path - great for HD800, OK with the PM-1, but not really desirable with the LCD-2 rev.1's already shelved highs. It's very slight, however.
Mike