I'm sure the main points discussed below have already been touched upon upthread, but here goes a summary anyway:
No I have done it before and i tuned them again today, i am just trying to see what the effects would be of tuning down an octave
Tension is halved for every half-octave (tritone) decrease in pitch. That is to say tuning a given string set down a whole octave, on a given instrument, gets you 1/4 of the initial tension.
1) What it does to the neck is, therefore, about the same as leaving only one string on it: heck of a preventative measure, highly overkill. If, after an accurate setup, you have reason to believe your neck can't take the tension of a particular string set, tuning a half-step or a whole step down would make sense, but not an entire octave.
[By the way, which specific La Bella flatwound set do you have on? Bass necks are usually designed to withstand very heavy sets in standard tuning: regardless of make, just be careful with the heaviest flatwound sets around, especially ones with heavy G and D strings = 55 and 75 gauge (respectively) or thereabouts; gauge of the low E is less important than you think.]
2) Steel-core strings have an operating range because of the inherent properties - tensile strength and stiffness - of the steel core itself. Tuned too high, on a given scale length, they break (uhm, let's not go there); tuned too low, they begin to have intonation problems (you run out of range at the bridge for the saddle to be set back for correct compensation), and the pitch gets wobbly with even the lightest plucking force. Which means, for octave-down tuning to happen, you'd need suitably large gauges for it, not normal ones (no matter how stiff and flatwoundy). Believe it or not, "optimal" gauges for contrabass E0 tuning are close to double those you use for standard E1 tuning. Yep, you read me right: about 200 for the E, about 160 for the A, et cetera. They do exist, as made by manufacturers like Kalium Strings and Octave4Plus, but they're few and far between in general. (And still the bridge may need repositioning, holes be drilled larger, and so forth.)
3) Depending on your tone goals, and for more justified reasons than keeping the neck safe, octave-down E tuning - again, with appropriate gauges - might be worth exploring.
Just be aware that:
- normal bass cabinets can't remotely handle that extra octave, unless you have a sharp high-pass filter (HPF - i.e. a bass cutting filter) applied upstream. Many bass heads do have an onboard one, but how well - if there at all - they work would remain to be seen in the specific case. Lacking an adequate filter, you need to get a dedicated HPF pedal (or program a virtual filter in a digital sound processing unit), or you will be blowing speaker cones left and right;
- you know when you play an uninverted C major chord at mid-keyboard on a piano, and then try to play the same voicing to the lowest C on the keyboard? The latter sounds much worse than the former, doesn't it? It's because of a phenomenon called "critical bands", linked to how the basal membrane in the inner ear works (and to how the signal coming from it is processed by the brain). In short, even consonant intervals become too narrow in the lowest register, and are perceived as rough by the listener.
Well, the same thing, to an extent, happens to the individual harmonics of a single note, if said note is lower (which the lowest 5 notes of a contrabass E-tuned bass guitar would be) than the lowest keys of a standard 88-key piano. In other words, notes below A0 have an inherent roughness to them. This does not mean you can't make meaningful music with them - there are extreme metal bands that make them work - but it does mean it won't be "pretty".
