Tech21 VT-Bass - where's the magic ? Found this thread where somebody plotted the response from the pedal at different settings. The character knob works on the mids and adds harmonics apparently.
Tech21 SansAmp VT Bass technical review Also from this thread: "Believe it or not, Mid and Character are almost the same control. Frequency wise there are some differences: Mid boost is set to 865Hz, Character boost is 1462Hz but Character and Mid cut overlap perfectly: 880Hz.
So why do they operate differently? The reason is the position in the circuit. Character is
before distortion, the low/mid/high controls are
after distortion.
It is a simple but very effective solution probably inherited directly from the SansAmp Classic. In practical terms the Character knob pushes the saturation forward (or backward) on the midrange. Because the mid control is tuned in the same area, the tone effects of the Character control can be compensated post-distortion if the mids are too strong.
The drawback is that the Character control can generate a lot of noise if turned up above a certain level."
So apparently to replicate the character knob you'd want to add an EQ block centered at 1462 Hz and play around with level and Q values before the amps in Helix.
Looking at the plots from the character knob sweep it seems like the knob at 9 o'clock has a spike around the 1462 Hz mentioned above. As you sweep to 11 o'clock the peak is lower than at 9 o'clock by -10 dBu and at 12 o'clock it drops -21 dBu below the 9 o'clock position but there's a spike at around 2.5 kHz of 13 dB.
So from my understanding of those threads a wide Q or maybe even a shelf at around 1.4 K to dial back the high mids before the B15 preamp could get you in the ballpark maybe? Or a cut around 2.5 kHz.
Maybe somebody else can chime in with how they understand those graphs because I'm pretty sure it could be replicated.
Paging @JKos since he did a wonderful job with the Noble DI in this thread a while ago.