The thing about balanced cables is that you will have a hard time to notice anything than double the volume. If the amp is really good, you can squeeze a few more dB dynamic range (which results in better THD+N, for example). That is if you couple a double headphone output (which also doubles the output impedance). However, that effect is absolutely minimal. Especially if your amp is powerful anyway, there is no way for you to hear an improvement. Considering the PM-3 is so easy to drive, I don't think there is anybody who can actually hear a sonic improvement.
As for the Opus #1, I reviewed it and I tried the balanced output with CIEMs. The balanced output circuitry is not the same as doubling the 3.5mm output, so effectively you lose the marginal benefits I just explained earlier. It's not a dynamic sounding amp and the balanced output will not fix that. There is only one reason to use the balanced out with the Opus #1 and that is for hyper-sensitive IEMs. Because the balanced output has a lower output impedance.
For the Oppo, I would not even bother.