Nope, that's not going to work. Manufacturers don't play by the 1/8th rule (or even understand what "output impedance" means, most of the time). You can find tons of cheap headphone amps (ex most TPA6120 implementations) that have 10+ ohms output impedance and rated for 8-200 ohm headphones, etc - not even close.I picked those based on previously read reviews and until recently never wondered what is the output impedance of them. I would like to know that to anticipate how will they play with future sets.
Can one tell just by looking at the linked specs what would the output impedance be @1 KHz (how most manufacturers are reporting it)?
For instance, I would think that for Bravo where it says "Output Impedance" one in fact should read "Supported headphone impedance" and seeing a minimum of 20 Ohm I would think that output impedance ~1 KHz would be ~2.5 ohm (x8 rule)?
Similarly for xDuoo TA-01 that would be ~2 Ohm?
Are these assumptions correct or I've got it all wrong?
Maybe the more knowledgeable of you would have a clue.
Components can give you a better idea of what might be going on (TPA6120 for example can be very low, but for heat and stability concerns, recommended implementation has a 10 ohm series resistor on the output, so >10 ohms output impedance) but that still only works if the amp maker followed the suggested implementation.
If your amp works in open loop, and you have an AC capable multimeter (DC won't work, need to measure Vrms) a simple way to get a ballpark figure is playing a 1kHz sine wave and measuring open loop voltage and voltage across a sense resistor of known value (in the general range of a headphone... where lower will give you a more accurate result iirc), then calculate:
Z = Rs * (Vo - Vs) / Vs
If it doesn't function in open loop, well then you need multiple load conditions and a slightly more complex formula to calculate it that I don't remember off the top of my head, but the idea is the same, you're intentionally constructing a voltage divider out of the unknown element and some known elements and recording the difference to use to calculate the unknown element given that a voltage divider was created and its outputs measured.
To get a complete curve instead of just a nominal value the same process can be done at many different values, but usually an automated tool connected to a setup with a known sense resistor is the way to go for that.
PS: a quick Google search: Xduoo TA-01: output impedance 7 ohms
Can't find an exact value on the Bravo Ocean, but probably in the 10 ohm range too, since what I did turn up was a lot of comments about it's "very high output impedance" being unsuitable for headphones under 300 ohms.
(that's with stock tubes though. I have no idea how tube rolling might effect things, I think it depends a lot on the actual implementation? but opamps can range from like 0.01 ohms to 10 ohms or so? )
PPS: If you can live with 1-2 ohms output impedance and don't need it vanishingly low, voltage dividers are your friend and can sort out linearity and noise issues on a whole variety of setups as long as you have the volume headroom to use them.