Thinking about this some more, it seems possible, perhaps even likely,
that you started your ardour session with a previous version of the
plugin and then updated CAPS. If ardour saves a plugin control map
with the session (likely needed for automation purposes), this map, if
using numerical indices instead of port labels, will not match a later
version of a plugin with reordered controls.
Depending on what version of CAPS you started with, what is presented
to you as the tonestack model selector could well map to the amp's
gain instead, leading to the result you described.
If this is correct, removing all references to the plugin from your
session and then reintroducing it should fix the problem.
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