You don't always have to high pass, you can use low shelves. You can have some guitar sound happening low, sometimes a mixer can have too much bass in his bass tone, which gets in the way of the guitar and makes the guitar sound bassy.
As for how far to go with a high pass is up to you. Try lots of places to put the high pass, listen to what sounds best and leave it there.
I think messing around with amp sims is a good way to get an idea for how much bass a distorted guitar really needs. Just find a metal preset and listen to what sort of bottom end the tone has. Is it enormous? No way. But it probably sounds really tight and full of upper low frequency info, and some lower mids. If it is a preset, it is made to fit the most common situations, which is in a mix.
Also I regulary boost with a bell at around 8k on my distorted guitars. Sometimes 4 - 8 db, not normally up to 8 db though. Not always, but many times it really helps my heavy guitar sound, so don't use your low pass set at 8.5 as a rule.
Just my 2 cents :Spin: