Focusrite claims <10Ω on the output impedance. I am not sure if it's the line balanced output or the headphone output impedance, but I will note this in the next few measurements that I make.
Worthy of note is that these are not hi-fi devices, but rather are purpose-built to be reference sound. It is their design to match sound as close to what it should sound like; as such I am currently assuming that the drive circuitry should reflect that.
Well, the point of "high fidelity" is originally reference sound (minimal changes to the signal) as well. In any case, "pro audio" hardware is not inherently more transparent or neutral than competently designed "consumer" devices. Even the HD audio codecs on PC motherboards have flat frequency response and low distortion (but often higher than ideal noise floor) on the line output once any unwanted software processing is disabled. The main differences are in features, connectivity (e.g. having balanced inputs, microphone pre-amplifier with 48V phantom power, etc.), and bundled software.
However, bad headphone outputs can easily be non-transparent, often because of having too high output impedance. Even 10 ohms can be too much for example for balanced armature IEMs, but for your particular headphones, which have relatively high and not very reactive impedance, it is not a major issue.
On the other hand, if the balanced output on the Focusrite is intended to be used as a line output only (which I suspect it is), then using it to drive headphones is definitely a potential source of problems, such as the obvious bass roll-off that can be seen on the frequency response. Line outputs are designed for high impedance (>1000 ohms) loads, and may have high serial resistance, and/or use small output capacitors.
If you want to test with the Focusrite only, then it would indeed be a better idea to create a balanced output from the two channels of the headphone output by inverting the signal on one of the channels. Even then, the doubled output impedance will change the frequency response slightly, but not as much as a typical line output.
In the end, whatever results you get, they are unlikely to show differences that are inherent (and audible) to balanced vs. unbalanced outputs. With normal dynamic headphones, either can be audibly transparent without much difficulty if well implemented. When a significant difference does exist, it is very likely because of some specific deficiencies of the particular device(s) being tested.