Just a quick heads up: if you have not done this before, try it first, perhaps with some other amp and mic that you have around.
For a well-practiced engineer, recording a few responses and creating IRs can be a task of 5 mins, but if you’re doing this the first time, set maybe 1-2 hours aside and also do some listening tests until you’re confident you can do this later under time-pressure in the studio.
“aliki” is available on most GNU/Linux distros, but its UI is somewhat idiosyncratic.
The overall process is
- Generate a sine-sweep
- Play the sweep into the amp, record the output
Deconvolve the recorded sweep to create the IR
- Edit the IR, (cut initial silence. trim long near-silent tail, apply gain)
Apart from aliki, there are a couple of other tools that can be used for this, e.g. jack-ir. And there are also proprietary tools for Windows and macOS.
After you have created the IR, load the file into a convolution plugin (convo.lv2 or lsp-IR or Klangfalter or …) and test that it matches your expectation.
In theory nothing. Amplifiers, speakers, cabinet and mic are all linear systems. However amps are not perfectly linear.
You’ll also record the guitar DI (after the effects and pre-amp) instead of the mic.
Again, in theory there should be no difference when using a convolver to emulate the rest of the chain. In practice however there may be subtle differences.
You could use existing tools (like guitarix), some EQing and/or use existing IRs:
perhaps http://www.osirisguitar.com/wp-content/uploads/7deadlysins%20Impulse%20Pack.zip to match it as close as possible.
or just live with it and create a new sound that works within the mix.