With the 4 cable method (4cm) you can connect your Boss GT-8 /GT-10 to your guitar amp, that you can use the preamp of your guitar amp. This is especially usefull when using a good sounding tube preamp, because then you can still use it. Another advantage of the 4 cable method is that you can switch between your built-in tube preamp and the preamp models of your Boss GT-10 / GT-8. The basic requirement for the 4 cable method is that your guitar amp has an effects loop.
Of course you can use the Boss GT-8 or GT-10 like every other multi-effect device from the '90th. Connect it to a clean guitar amp and only use the available overdrive and distortion models, but not the preamp models. With 30 different overdrive and distortion models - in the '90th I had an Ibanez PT3DX with 6 overdrive/distortion models - you should be able to get every needed sound from blues to rock to metal.
Here we'll use the guitar amp models of the Boss GT-10, because they simulate our none existing guitar amp. In doing so we'll notice that the guitar amp models are a bit thin sounding. A lot of highs and only a few low mids is not what we expect from our guitar through the Boss GT-10 or GT-8. But, in fact it's okay, because when listening to our favorite artists albums we can recognize a nice transparent sound, too.
This is achieved by reducing the lows and low mids of your guitar.
We can connect our Boss GT, Vox Tonelab or Line6 Pod to an already existent guitar amp. Furthermore we can use an full range system like a PA or keyboard amp. But we must pay attention to the correct output setting. When using a guitar amp which always colors the sound we should plug the modeler into the effects return of our amp instead of using the guitar amp input.
A lot of us think about a guitar amp as an essential part of our sound.