Is there any guitar setup that grindcore guitarists (bands like Pig Destroyer, Wormrot, Magrudergrind, etc.) use so that there is no feeling of absence of a bassist? Is it just that they tune the guitars low, or is there some other setup?

I know a bit about Scott Hull's setup for Pig Destroyer.

Tune low (drop Bb), utilize the ability of the tuning to transition from any type of riff into something filled out by power chords on the bottom end.
Bass heavy cabinets on both sides of the stage, some of them turned sideways. I believe Hull uses Ampeg, Emperor, and Mesa cabs.
Thicker sound from splitting the guitar signal to multiple types of amps. Looking at pictures I took at a PD show, I don't see duplicates of anything in the backline, it looks like two very different Mesas, one with an Ampeg 6x12 cab (bass cab?) and maybe a Mesa cabinet on the other side.

Rather than aiming for specific gear, figuring out how to diversify your playing to fill out everything that isn't drums or vocals while being interesting is the most difficult part. Both live and for recording, mixing two different tones will help fill out the sound. I think a large part of it is exploring different and unconventional/uncommon options.

I suppose there's also the option of buying an octave pedal. I'm not familiar with them myself, but I've seen them used pretty convincingly.

I've heard of some companies selling custom guitar strings that are designed for turning an octave down, I imagine they could be quite useful.

In studio settings, having a low-heavy third guitar panned in the centre should beef the mix up enough to compensate the lack of a bassist.

Instead of having a bass guitar, you just have a regular guitar tuned like a bass guitar? That would be a mess to mix with most guitar pickups, and going to the trouble of setting up a guitar to make it not sound like shit sort of defeats the purpose of not using a bass. I'm aware that many 8-string guitarists tune their 8th string down to E, but that seems like an impractical, roundabout solution.

I don't imagine they'd sound that bad, it's a similar concept to those Fender Bass VI guitars, though they're not very common.

7-strings down-tune quite easily and keep tune. I regular tune to G#/Ab and it holds tune on my ESP Ltd. MH-417, though that is almost pushing it with stock strings on mine. Mine has a 25.5" scale and comes standard tuned to Bb (like Hail Of Bullets uses, for example). Guitars with even a longer scale can handle it even better and go even lower. Bands like Abysmal Torment use said G#/Ab tunings which is super-low. I think you should consider a 7-string or alternatively a baritone 6-string (Abysmal Torment uses one, an ESP, for ex.) so you can tune low enough to get the sound you want. Most BDM/Slam/Grindcore and many DM bands down-tune, anyway, bass or not.

A great thread on various tunings, bands that use them, and which tunings can be done for which guitars might be helpful and it can be found here: http://en.wikipedia.org/wiki/Guitar_tunings#Lowered

Have the PA-guy boost the guitars around 100 Hz. Or do it yourself with (parametric/graphic) EQ.

That's a bit misleading, since it often lists tunings that a band rarely uses.
Morbid Angel tuned to Eb on almost everything, which would have a low Bb on the middle-era albums where Trey used a 7-string (F/G/H), and occasionally dropping the bottom of the six strings to C#.
Cannibal Corpse and Slayer mostly tuned to Eb, using Bb somewhat in later years.
Dream Theater only tuned to Bb for one song, Panic Attack. John Petrucci keeps guitars tuned to E, Eb, D, drop D, C, B(7-string), Bb, and A.

This thread is a good resource on tunings, though a few of the lesser known bands aren't completely covered: http://www.ultimate-guitar.com/forum/sh ... p?t=797585

I agree, it's not a be-all-end-all for which bands use which tunings, but it's a good reference for the tunings themselves. Another good band-tuning thread which is much more DM oriented is here: http://www.riversofgore.com/pit/showthread.php?t=105 . It breaks down a lot of bands and their tunings relative to specific albums. A lot of bands, as you alluded to Zodijackyl, change tunings over different points in their careers or album to album.

This.

Get a Morley (or other brand) A/B-Y splitter and run 2 separate rigs. One for a standard guitar sound and one for your "bass" sound. An octave pedal for the "bass" rig would be good as well as using a bass cab and would allow you to not tune so low on your guitar or you could use it sparingly and only kick it on when you want shit to be SUPER heavy.

My opinion on bands with only guitar is that your sound needs to be MONSTROUS but still needs to be concise unless you're just going for a wall of distortion. The bass is often what cuts through for really noisy bands and kind of defines what's happening so if theres no bass you need to make it a point to have your riffs be either really discernable through the noise or make them simplistic enough that they don't get lost in a giant wall of noise.

Good example of discernable riffs
http://uneasylisteningrecords.bandcamp. ... ient-stone

Good example of more simplistic approach
http://powercup.bandcamp.com/