Re Axoloti:
Is it transparent sounding as a volume control?
I think so. In my setup, it's always on, so perhaps I've compensated with other settings.
Can the volume curve be adjusted?
Yes, although I haven't done it. There will be several different ways of varying complexity.
What else can it do? FX?
Basic guitar FX. I use delay/chorus/flange and some bitcrusher distortions.
It appears they are out of stock.. Do you know if this is a typical thing, a temporary thing, and how long this condition might last?
It's a one-man operation, and he makes batches from time to time. You could post a message on the forum asking about the next batch if you wanted.
Are you active in the forums there?
Not active, just a browser. Now that I've got it doing what I want, I'm not changing it. I have bursts of work on it (adding new patches as the music demands).
Do note that it has low-impedance line inputs, not instrument inputs.
Yes, I have it between two buffered pedals.
If you do decide to consider it again, you can download the software and get a feel for how much it can do - without the hardware. There's an active community of people developing all kinds of good stuff. Much of it is more to with synthesis than guitar processing, but I've found some useful things that would be very hard to find in an off-the-shelf pedal.
Hope that helps.