A lot of times I read about the bad overdrive and distortion modeling of the Boss GT series. But what's wrong with the overdrive / distortion block?
Everyone of us using real amps and distortion stompboxes knows that you cannot combine every distortion / overdrive stompbox with every amp. E.g. a shrill distortion stompbox doesn't sound good with a Fender amp.
You almost get one answer when dealing with the question about the signal level: "Use the highest possible signal level because otherwise the Boss GT-8 and GT-10 analog / digital conversion will produce undesirable low quality results (digital aliasing)." Simultaneously you are pointed to the fact that you must not overdrive the input of the Boss GT AD-converter because then you'll get a lot of digital distortions.
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.