So some devices like Laptop charges use electricity, however small an amount, when they are plugged in and switched on at the mains but not in use.

What about Guitar Amps? Couldn't find this on Google but, if I leave my guitar amps (15W Marshall and 15W Line 6) plugged in and switched on at the wall, but the actual amps are off, will any power be consumed, and if so, a significant amount?

A very small amount but greater than zero. Laptop chargers and pedal power supplies always use some juice and generate heat when plugged in so better to put them on a switched power strip and switch it off when not in use.
"Your sound is in your hands as much as anything. It's the way you pick, and the way you hold the guitar, more than it is the amp or the guitar you use." -- Stevie Ray Vaughan

"Anybody can play. The note is only 20 percent. The attitude of the motherfucker who plays it is 80 percent." -- Miles Davis

Guthrie on tone: https://www.youtube.com/watch?v=zmohdG9lLqY
Alright so it's not really something to worry about? It's just with this new furniture I need to cover up the wall power outlet for the two amps I have for them to fit where they are so I won't be able to access the switch. Thanks!

Yeah it's something like laptop chargers converting AC to DC or whichever way round, so they always generate heat and of course laws of thermodynamics etc.
The power switch on many (most?) amps physically isolates the mains from the unit, so it's likely that no power is being consumed when they are switched off.
“Ignorance more frequently begets confidence than does knowledge.”
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What about Guitar Amps?
It's this way with every device that has a transformer first thing after the power plug.

Transformers aren't perfect, so they suck a bit of current if they're plugged into something that puts out alternate current.
Not much though, nothing to worry about.
Name's Luca.

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Clue: amplifiers amplify so don't turn it on if you need quiet.
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It's this way with every device that has a transformer first thing after the power plug.

Most (99%) will have the power switch between the supply and the transformer.

(Actually many amps today used something called a switched mode power supply rather than a transformer, but the same applies, the main on/off switch will disconnect it from the household supply)

(An exception to this would be a small amp powered by a wall wart)