And he still did record the bass lines on their latest album, even though he's not in the band. They state that they won't replace him, as he's an integral part of their sound, which I totally agree with.
Doubling the guitar is exactly what the bass does in most djent type music. At least you can actually here Nolly. Some of those bands you wouldn't even know there was bass on the recording at all.
This exactly. I have the STEMS from Periphery's last two albums. If you listen to the isolated guitar and bass tracks, you can hear what they're doing. Nolly generally doubles the guitar part, unless their doing some twiddly-diddly more lead oriented melodies. Then, he's basically all you hear holding down the rhythm, and it still sounds huge, because of how he's engineered his tone.
The guitars are actually very anemic sounding by themselves, as they lack any low end. That's by design, so the whole thing doesn't turn into mud.
I'm a fan of the band, but I know that they, and this style of music is not for everyone. If you listen to the way they produce it, and understand what's going on sonically, there's allot that anyone can learn though. These guys know what they're doing, and do it well. That's probably why so many other bands copy them.