As the time goes, I see new bands using lower and lower tuning. For example if my memory serves me right, Torsof*ck uses octave down, band called Batushka uses f# tuning. How are they doing this? Don't you practically need bass string on a guitar?
P.S I'm not intending to do that myself, One way to do it without directly tuning is pitch shifter.
Tuning a guitar lower than standard will certainly impart a darker, heavier sound. That's the reason many metal bands seem to employ various type drop tunings.
A standard scale 6 string guitar (25-1/2" or 24-3/4") tuned down an octave (or tuned down to F# below the standard low E) would not be very practical to play in a typical guitar playing sort of way - due to the floppiness of the strings. You can get acceptable results playing power chords on the bottom three (fattest) strings by using heavy gauge guitar strings. But any full chords using open strings would sound very muddy and not very crisp. And the feel would be super sloppy on the thinner strings.
But for heavy metal music, power chords on the fattest strings might be all you need!
Many bands who use lower tunings (such as Kaki King, System of a Down, and Metallica) utilize what is known as a Baritone Guitar. Examples include the Epiphone Robb Flynn Baritone Flying V, or the Schecter Ultra-VI Baritone (see pictures below). These guitars have longer scale lengths so they can reach the lower pitches while maintaining a playable string tension. Most Baritone guitars have a scale length anywhere from 26" to 30". Electric Baritone's often have pickups that are specifically optimized for giving the lower notes a clearer cleaner response.
A 6 string Bass which would have a scale length of around 34 to 35 inches can be tuned like a standard guitar although an octave lower - but chords with open strings are still going to sound muddy.
Also popular with Metal guitarists is the 7 string guitar which has an added bass string designed to be tuned lower than the standard 6th string.
Although a Baritone guitar is capable of lower tunings (and would be much more playable in an octave lower tuning than a standard scale 6 string) - most players tune their baritone guitars either B to B or A to A (4th or 5th lower than standard) and can get away with playing regular guitar chords without too much rumble. Certainly if you insisted on a full octave low tuning, you would get much better results with a longer scale baritone guitar than you would trying to modify a standard scale six string.
To set up any guitar for super low tuning (like F# below low E), you will want to use the heaviest string gauge possible. That may require larger nut slots. I recommend having them altered by a qualified professional guitar tech or luthier because if you accidentally make them too wide you pretty much need a new nut.
Lower tension strings are going to have a wider oscillation envelope so you will need to raise the action. You will have to make adjustments to the truss rod anyway because with less string tension on the neck you will need less compensating tension on the truss rod.
Finally, whenever you make radical changes to the string tension, you will need to adjust the intonation at your bridge.
Below you will see pictures of the aforementioned Baritone Electric Guitars.
Epiphone Robb Flynn Baritone Flying V
Schecter Ultra-VI Baritone
As you detune strings, they start to get "floppy" and the guitar becomes hard to play well. To prevent those problems, you can use thicker strings, a longer scale length, or both. At some point, the strings become so thick that you need to widen the nut slots or else the strings won't sit in them correctly. You also may find that you need a higher action and string spacing may get tight.
That's part of what makes a bass guitar different. It uses a much longer scale length, wider nut slots, heavier gauge strings, and wider string spacing.
If you compromise a little on the floppiness factor and the string spacing, and start off with a guitar on the longer end of scale length, I'm sure you can make it mostly playable with thicker strings, higher action, wider nut slots, and an octave down. It might not be feasible to restring such a guitar for standard tuning, though. At least not without getting a new nut and making the reverse adjustments to the action.
I play a lot of super low tuned stuff, I'm also a bas player and I've had a lot of experimentation. I once had a 135 bass pack on a standard American strat guitar. Sounded terrible BTW. however there are ways around low tuning problems.
I currently have the strat in F#, so nearly a full octave down, and a baritone in drop E. the baritone sounds amazing, now I did some tweaking to the strat to accomplish this. Off the shelf it sounded awful anything below drop C-B range. I filed the nut out, drilled 2 of the tuners wider and got a pickup modeled after the Seymour Duncan Nazgul series.
I got an 85-18 set of strings from Stringjoy.com and it sounds great. You really need the massive strings but even an 85 in E would work and a 50 dollar pickup designed for heavier tones. It's still a 26 inch scale and the necks holds up fine.
The real problem is your bass player. If you has a bassist or want one in the band at all, low E0 is hard. You're talking about absolute necessities for the bass being at least a 36 inch scale probably expensive bass, definitely some active pickups, strings in gauges that is difficult to find and will cost a fortune. So getting a guitar down an octave isn't that bad, the bass getting an octave lower is nearly impossible without a whole rig and instrument upgrade and a lot of money.
Being a bassist myself, if a band wanted me to play with them and they said they tune down an octave I would just walk away. I never tune below A0 for a bass because of the problems, so if you wanna play heavy stuff and not screw the bassist over, id say the lowest you should go is G# or A, depending on the rig. Most 5 string-basses can handle low A, and G# is even possible with the right amp and some Stringjoy 145 guage strings.
Drop A still sounds fine with chords too so it’s a good happy medium of not needing heavily modded instruments and not losing chord capabilities. I only have guitars tuned in F# and drop E because I used them for solo projects where I essentially AM the bass player. Animals as leaders does that. its not great for super heavy stuff because without the bassist everything feels flat and hollow, but for some music its fine, more ambient and tapping works without a bassist.
Now if you absolutely need that low E tuning, and modding isnt an options, lets assume there is already a band with a bassist an octave down and E is what you MUST have, you totally can just use a bass and run it through a guitar amp. crank the highs and roll the bass knob down to nothing, keep distortion moderate and run the bridge pickup, play with a pick and you essentially got an 8-string alternative on a budget right off the shelf. You can even buy like E-a-d-g-c extra lights and put them on a 5-string bass and they will have that looser oscillation sound that works well for Djent or metal. a 90 guage string on a bass will sound as good as a 90 on a guitar and it will have more clarity from the longer scale.
lastly, if you get a line 6 Shuriken guitar, you can buy regular strings and it digitally tuned down a full octave without losing tone quality. 12 ft ninja uses that and they play an octave below drop D in some newer songs and that’s with a set of like 48's physically tuned to regular drop D. and that beast sounds amazing. its also like well over a grand just for the guitar alone, and 500 for a pedal. so ideal, but not cheap.
