I agree with Mike, a capacitor should do it. Just turn-down the guitar amp before you start, because the signal will be on the "hot" side.
If the signal is too-hot, so that you always have the volume set to '1', let us know. There are easy ways to knock-down the signal with a couple of resistors or a potentimeter (volume control) on the Arduino's output.
And I was also wondering if sending a tone to the amplifier would be the same as sending a tone to a peizo speaker.
Yes... But LOUDER, and of course the guitar speaker can reproduce low-frequency tones that the piezo cannot. What would your guitar sound like played throught the piezo?
The pitch will be the same (at frequencies the piezo can produce) but the tone/character will be different.BTW -
Be very careful with high-frequency tones... Say above 10kHz... You can end-up generating lots of power that the speaker can't reproduce well, and perhaps above your hearing range. So the amp could be generting lots of power without you being aware of it. Guitar amps are usually rugged and it will probably survive, but it's something to avoid.
For the similar reasons, be careful running "test tones" into a high-power hi-fi system. You can burn-out almost any tweeter with a constant 100W 20kHz test tone. And, because our hearing is weak at 20kHz (if we can hear that high at all), you might not be aware of what's going on 'till it's too late.