I use a Mackie Big Knob. I love it for its convenience. I can connect multiple sources, multiple outputs, and select amongst all of them as I want. I can have three headphone amps connected, as well as three power amps or powered speakers (could even connect an HE6 to a power amp and control the volume with the Big Knob). And be able to select between 3 sources (and one sucky phono pre as the fourth) to any of the outputs, and even have multiple outputs gong at once or multiple sources being summed and going at once. Really nice for convenience. And it handles both balanced and unbalanced inputs and outputs.
The downside is that the sound through the Big Knob isn't transparent. You do lose a little bit of stereo separation and a little bit of openness in the sound. It still sounds fine. But if you start playing with $1000 DACs, $1000+ amps, and $1000 headphones, the Big Knob will be the sonic bottleneck. I'd say that up to about the level of the HD600 and a Schiit Lyr and a couple hundred dollar DAC the Big Knob is totally fine. Start playing with the big toys and the Big Knob will limit the capabilities of the other gear.
One way around that would be to use a DAC that has two or more switchable outputs. Have one of the DAC outputs go direct to your good headphone amp and the other to the Big Knob. And use that direct connection for the good sound with the good headphones. Then use the Big Knob for everything else.
Another downside is that TRS jacks in the back of the Big Knob can get flakey and can start getting some poor connections after being left alone for a while. You'll hear a problem and then have to pull out all of the cables, blow canned air in each TRS jack, then plug everything back in. TRS or TS jacks aren't the most reliable way to connect audio gear.