Debating between Benchmark DAC3 HGC vs. RME ADI-2 FS as a headphone amp and DAC via USB input to use with Qobuz wasapi exclusive for streaming. My specific headphones are Planar Magnetic Headphones with an Nominal Impedance of 26 ohms. Would I be better to use DAC3 HGC as headphone amp or select something like RME ADI-2 DAC FS, given my specific headphones. I do not want to add any additional headphone amp equipment, given how expensive either piece is.
Might be late, but as another RME user chiming in, go for the RME. It has comparable performance to the Benchmark in nearly every tested metric, save for the intersample headroom. That being said, the RME itself has some headroom, but neither have what I would consider "enough" headroom. You're better off using ReplayGain with oversampling to detect intersample clipping.
The RME also has a plethora of DSP features that practically no other device has, including PEQ, which suffices for room correction for speakers or to flatten out headphones. It even has the same pro-level outputs at +24 dBu. Its headphone amp is both less noisy and more powerful than the DAC3 HGC's headphone amp, and can do 1.5 watts per channel into 16 ohms.
If you get the ADI-2 Pro, you get 2x the power for headphones, if you're willing to use an adapter cable. That's just shy of 3 watts per channel, which is a lot.