I had to capture this again so it might not be exactly the same as the IR I posted earlier but it'll be very close.
View attachment 66641
I remember the lower frequency rolloff being steeper previously but I can't replicate that. Regardless, it's a pretty unexciting curve. This again is at unity gain with little to no overdrive. Increasing the output on the Centura if anything only flattens the curve.
Introducing some gain does change the curve though. This is with the gain at around 3 o'clock (so more than most people would use):
View attachment 66643
IIRC the gain pot as well as driving the circuit more, also blends in/out the clean signal. So what we see here may be said to be closer to the 'natural' tone of the Klon, though as I mentioned it's at settings few people would use.
I still think it's the buffer that makes the Klon what it is, and we can't model that so that might be the reason for its absence (that said, the Helix has one that is apparently very popular so maybe I'm wrong). But try the IR I attached previously. It sounds nice after a FET boost with a Strat and a clean amp.