Linear, is something that describes the amps response. Flat, or not accentuating any frequencies over the rest. Speakers, also aim to do this. But then it sounds sterile, and bad, for guitar. Guitar amps are actually hideously unlike this, and sound yummy for it. Look at response curves for bespoke guitar speakers...not linear.
Atomic make tube power amps, that make your pod louder, but with a sprinkling of this "incorrectness". So you model upfront in the pre-amp, and then power amp with a bit of mojo and breakup and speaker distortion. This is meant to make your digital righteous.
But i think your question is really about what would a guitar sound like going into an ultra linear amp and speaker setup. Like a nice home stereo?
Well, because of the flat response of the system, and the limited range of the guitar signale, it would sound thin, boring, lifeless. Like arse. You NEED a midrange hump.