Not mandolin, but someone here should know....

[Paul Busman]

We saw We Banjo 3 the other night. By all means go see them if they come anywhere near you. One of the guys even played mandolin (with Tone Gard FWIW )
They don't have a bass player, but at times I could clearly hear bass lines. I noticed that they coincided with the acoustic guitar player's fingering of the low strings. I couldn't help noticing also that he had two amplifier cords coming out of the guitar. How did that work? It was quite effective.

[foldedpath]

The cleanest way to do it is with a hex pickup in the saddle, taking the bottom two (or whatever) strings out to something like a Roland VG series processor and pitch shift down an octave. Or split out the signals and run to two different octaver pedals.

But there is another quick 'n dirty way that's been around for years. On a pickup (or mic) equipped guitar, you add a second pickup -- a slim humbucker like a Fishman Rare Earth -- and turn it sideways in the sound hole, so it's just under the bass strings. Then feed that signal out to an octave pedal to kick it down. it's just a low reinforcement that can sound pretty neat. It was an occasional fad among the old New Age fingerstyle crowd.

I tried it myself at one time, but was never happy with glitching from the octave pedal. I had a couple of electric guitars with RMC hex pickups feeding a Roland VG-88, and that was much cleaner for octave reinforcement on the low strings. But the sideways humbucker idea is easier than a full hex pickup installation.

Here's a video with a guitar player using the sideways pickup for bass reinforcement. He starts with it turned off, then kicks it in around the 1:20 mark for the rest of the clip:

[EdHanrahan]

IMHO, the perception of bass-vs-treble can be highly subjective.

Consider that:
- Most fretted instruments have a 3 1/2 octave range.
- Bass instruments (voil or guitar) are tuned only ONE octave lower than guitar (two lower than mandolin). That's out of the 8 or so octaves that older human ears can hear.
- Bass players spend a fair amount of time playing above their lowest octave (maybe not so much the BG ones, but...).

So it shouldn't be too surprising that a guitar can "sound" like a bass even without resorting to specialized hardware.

Two cases in point:
- In a local Italian music quartet, my buddy simply turns up the mixer EQ's "bass" for his pickup-equipped, and mostly finger-picked, nylon-string classical to get a pretty convincing bass sound.
- In a recent jam with three mandolins, one banjo, and a good-ol' bluegrass flute (lots of treble!), I took to playing some fully adequate thumb-picked "bass" on my (yes, bass-heavy) D-35, much to the agreement of all.

Given the subjective nature, and especially against a treble environment such as the OP describes, "bass" on guitar isn't too much of a stretch, IMHO.

[Tavy]

Cool effect, thanks for the vid!

[pops1]

We used to do that for a couple of years in a blues band I was in. Lost the bass player, had two guitar players so they switched off when one was playing lead the other bass lines. One with an octave pedal, the other just into a second bass amp. Worked well, we had a bass player in the audience one night come up and want to know how we had bass with no bass player. Wow just thinking about that, it was about 18 years ago. How time flies.

[Tim Griffin]

I saw them last year. The guitar player has a sound hole pickup, turned sideways to pick up the low string and it feeds an octave pedal separate from the main pickup.

[Paul Busman]

Thanks Tim-- that makes perfect sense. BTW- the fiddle player occasionally provided drum sounds courtesy of his right leg stomping on some sort of pedal.

[NEH57]

Fiddle - drum
https://www.youtube.com/watch?v=c2Rx2KSW3-c

[Jim Garber]

Enda Scahill (tenor banjo) is one of the very best Irish TB players. I would love to hear that band some day.