Tweaker 15 input stages..

[Helmholtz]

I wonder if part of the decision to use a SS Cascode is for the headroom like the EF86..

I don't think there is much difference in input headroom between an ECC83 and an EF86, both with bypassed cathode resistor.

Unbypassed cathode/source resistor increases input headroom but lowers gain.

I think the limited input headroom influences attack response and thus contributes to the typical tube sound/feel.

[teemuk]

The input stage is mainly going to receive very low amplitude signals, even at gain of x100 sufficient headroom shouldn't be an issue. Concerning headroom, I would be more worried about at which amplitude the grid of the second stage will start to conduct and turns to a diode. It's headroom is probably 50 to 100 times lower than that of source or plate of the input stage. ;-)

As noted previously, the reason you see a solid-state cascode there instead of a usual triode is because they can. With its extremely low signal input the stage won't overdrive so you will not hear a difference in that regard. But the semiconductor circuit has less noise and microphonics, and no filament hum, and that is important for a stage, which receives the signal of lowest amplitude that subsequently gets amplified by highest ratio of gain.