what are the differences in sound characteristics? does it sound fuller if it has more than one speaker?
Oh yeah it does.

Comparing my Bassman (with four 10" speakers) to, say, a Blue Deluxe (with one 12") is like comparing stereo audio to mono.

It also makes a difference using overdrive and other effects. Some amps with single speakers sound almost like the speaker is "trying too hard" to push out the desired effect. It's obviously hard to explain differences in sound with words, but if play around and compare, you'll notice differences.
Tobacco Sunburst Fender Strat
(Lollar Dirty Blonde pickups)
Fulltone FatBoost
MI Audio BoostnBuff
Fulltone OCD
This1smyne "Pearl"
Wilson 10Spot II
Red Witch Phaser
Fulltone ChoralFlange
Tech21 RVB
'59 Bassman Reissue
pushes more air, you have sound coming from more places with more speakers too, for example some half stacks 2 bottom speakers are aiming straight and the top 2 are slightly slanted
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