I've not owned a Darkglass equipped model, but I will say that the Glockenklang in my Super J does very musical and "polite" EQ'ing without any baked in color. If I set the EQ to flat and then bypass the preamp with the active/passive pull pot the settings are indistinguishable. It's not an EQ to use for flavor, or vibe, or whatever. It's a very neutral EQ for dealing with room or monitoring issues. It's very handy for me since I'm usually playing with IEMs.
Regarding the 18khz high band. I'm an audio engineer. I mix music professionally. I can say definitively that if you have an 18khz high shelf on a bass guitar it's not going to do anything audible. This leads me to believe that the 18khz high band is actually a peaking control rather than a shelving one, and particularly a peaking control with a very wide Q. It boosts highs well into the audible range, but just like everything else about this preamp it does it in a very polite way. This makes me think that the Q is wide enough to boost down to somewhere in the 4-6khz range at the lowest, and it definitely doesn't get as low as sub-4khz since that 1-4khz range is where all of the non-polite stuff lives up top. If I had to guess I'd say it's boosting from 8'ish khz to 18khz and then tailing off somewhere at frequencies only dogs and bats can hear. Since the physics of a bass have it naturally rolling off up top it ends up very musical. I rarely boost it. The real magic of that control is the passive tone roll-off when you cut with it. That makes it a REALLY useful control, for me at least.