Years ago I kept wanting to go below bottom-E, so I bought some Dewtron bass pedals which I could extend the range down to a C (sadly those pedals went, 'cos they'd be worth a few bob now).
Some players tuned their bottom-E string down to a D; I think John Wetton did that. For me that's a bit of a faff
I have a few basses with Hipshot drop-D eXtenders which can flip the E down to a D, effectively automating the down-tuning; they do what they say on the tin.
Kubicki basses had a flip-down lever to add two frets on the bottom string.
You can go down a whole string on a 4-string, so do BEAD; a number of bass players do this, including Ian Hill of Judas Priest, but you then lose the higher notes and have to transpose strings compared to normal tuning.
But all of those require doing something which makes a change away from just the normal playing of the instrument.
If you want to go low and keep full-range, you really need a 5-string; just play the bass and when you get to E you've got the extra string there, plus it's there all the way up the neck, so it's a natural extension.
There are a few issues with 5-strings:
- You need to shift away from the "bottom-E" mindset, as the E is no longer the bottom string.
- A set of strings costs more.
- The necks tend to be wider.
- Some basses seem to have a dull-sounding B string compared to the others.
- Your amplification might not be happy playing a low-B.
- Until you get used to it, you may struggle with swapping back & forth between a 5 and a 4; however, folks can play bass, guitar, acoustic, synth, pedals, whatever, so it's just a matter of getting over that hurdle.
My favourite 5-stirings are a couple of Warwick MIG bolt-on Corvettes; I was very disappointed with the Rickenbacker one (the only Ric I've sold) and my Status ones have really nice necks but suffer a bit from the dull-B; I've also got another fat-neck Warwick Corvette which doesn't suit my fingers. I've had a couple of RockBass 5-ers they were for me unplayable, also a Music Man SUB5 which was just big.
I don't see the point of a 6-string bass; I'd rather have a doubleneck with a guitar on it.
I'd really like a Chapman Stick to try and get my head around - maybe this year will be when I pick one up!