Hi there.

I own a Boss GT-Pro and a Boss GT-10 and I know that you can use the 4 cable method on these units.
The way I understand it is that you should be able to use the built-in preamps and have your pedals in the right order, or the order you like or like it would be if you were running you wet effects through the effects loop, and by doing this your amplifiers preamp should be turned off, right?.
So that the two signals won't run together, because that would totally ruin the sound..
But the 4 cable method will do that you can use the built-in amps and no have your basic amplifier turned on or running at the same time? Am I right?

I don't understand your wording very well.

Obviously you need to have the amp turned on and running. You are not bypassing the preamp on the amp (again, hard to understand what you are referring to with "the built in preamps"). If you wanted to bypass the amp's preamp you would simply plug the FX unit straight into the FX return. The 4-cable method lets you put FX in front of the preamp and in the loop without using two different FX units.

It does not totally ruin the sound, and there's no reason to think it would. It's just like running individual pedals and using the loop, it's just a different process with a multi-FX because you need to use a bit of extra signal routing.

Sorry if my wording is confusing..
I'll try again..
I wanna use the 4 cable method on my Boss GT-Pro and GT-10, and the way I understand the 4 cable method is that by connecting it that way you'll be able to, among other things, to use the built-in preamp models of the FX unit. Which will allow you to not use the settings/tone from your regular amplifier. So when you use the 4 cable method you'll be able to switch between your amplifier's sound, tone and settings, but also use the built-in models - without the two signals being mixed.
Is this better?
If you wanted to use the built-in models, it might would be best to run the GT just through the effects loop. So: Guitar > Amp Input. Amp Send > GT input. GT output > Amp Return. Just my opinion, no rules here.

Generally, you don't want an amp model in the preamp of an actual amp. Your real amp will "preamp" the amp modelling to the real amp's sound specs. Depending upon how the multi-fx unit is set up, the amp models might also be placed in the effects loop if you were to use the actual 4 cable method. Even then, you'll have a preamp signal from the amp loaded with distortion and other preamp pedals, preamped by the real amp, into digital modeling amps.

With my above chain, the preamp just messes with the guitar signal. Then it sends that signal to the multi-effects to work with. Hopefully your distortion models will work with the amp models reasonably well as if you had real distortions and real amps. Then the fully "effected" signal will be sent back to the amp. Then the amp just amplifies that effected signal, amp models and all, rather than doing sound shaping to it as the preamp would.
Ok, I get it now. Yes, all you have to do once you have the 4-cable setup is to assign FX1 and FX2 to footswitches on the GT10. Those are now your output channels. I think the default would be FX1 bypasses the amp's preamp and FX2 does not. Switching the preamp can be assigned to another footswitch. It might be possible to tell the GT10 to do both at once but I'm not familiar enough with its advanced operation for that.

Will's method above does not allow you to switch between the amp preamp and the GT10 preamp.
Place the active(turned on) S/R block in front of ch.B & select the internal "through" preamp model for ch.B. Choose whatever internal preamp model you want to use as ch.A. In the system output select choose 'Lines/Phones'' mode & turn off the speaker sim for ch.A. Turn the NS1&2 off as well if you like, they alter the tone IMO. Create an assign or select from one of the default basic assigns that allows the Ctrl 1 pedal to switch between Ch.A or B. That should do it.

There is another way of doing it but the one i mentioned requires minimum assigns to get it working. For connecting the cables between the amp & fx unit, there are plenty of guides, images, videos available if you use google, so i'll leave that out.
Thanks for all your replies!

So, after a lot of searching on the net I found this:


This is for the GT-10 - but will of course also work on the GT-100 and GT-Pro, I am sure!

And this really says it all - the answer to my question:
"This method effectively isolates your amp's preamp and power amp sections as independent components. You have the ability to place digital effects anywhere around them (EFX Chain), rather than putting the entire GT unit in front of the signal chain.
It also allows you to swap between using your amp's preamp, and other preamps that are in the GT."