@CARLOS VALINHAS:
Actually there is an option 4) that I didn't include above because it's a set I've never seen for sale anywhere. In fact, I've only seen it mentioned in relation to the instrument that it was made for, which is produced under the same brand as the strings:
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In practice, Magma Strings has announced - and is perhaps already producing, but I've no idea - a 5-string bass in standard guitar scale, the Transpositor Bass, equipped with strings designed under the same principles as the Transpositor series sets that I discussed in post #15, with an added low B string. The interesting thing is that the company has disclosed string diameters and tensions for the accompanying string set, for the purpose of comparing them to those of a conventional, long-scale steel-core set and bass:
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If you look at the second tab, last two columns, we have the following gauges and tensions converted in (decimal) inches and pounds:
G .04", 15.75lbs
D .053", 15.86lbs
A .063", 15.53lbs
E .095", 17.4lbs
B .12", 17.62lbs
This can be useful to you in two ways:
a) if you're expect the LaBella Silverback set to be too much of an increase in tension, you could get this Magma set (assuming you can order it from where you live) and simply not use the low B; or you could contact them and ask if the G-D-A-E strings in this 5-string set are the same as those in the GCT-E and GCT-BG10, if those are easier to find or more interesting to you;
b) if, conversely, the LaBella .115 gauge still looks thin to you, you could get this new Magma set, discard the G, install the B-E-A-D and tune them up to E-A-D-G. However, I'm only saying the option exists, not recommending the experiment: if you decide to try it, proceed at your own risk. (All this assumes these strings can be found/ordered at all; if they cannot the points are irrelevant.)
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Finally, in case you live in Portugal, you could just contact the AGI/NOVAGI/Dragão string company and order a custom set from them. They already make nylon-core bass guitar sets - here's an oddity, a set made for two-octave-down violin tuning (G1-D2-A2-E3):
Dragão D037 Violã Baixo Acoustic bass Bass Classical 4 string
but there's another one for regular EADG (long-scale, I guess) that has G:.048 / D:.065 / A:.093 / E:.11,
so it would just be a matter of winding them to the correct length for your 25.6" bass.
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(Of course if you do live in Portugal chances are you already know about them... ^^)
"...but there's another one for regular EADG (long-scale, I guess) that has G:.048 / D:.065 / A:.093 / E:.11," this is for 750mm string vibrating lenght. I ordered a set and i must try it to see what is the tension behaviour, but i presume that i can achieve some more tension because the linear mass of the strings are greater.
A big improvement over the stock white rubber strings in both volume and tone. Slight increase in finger noise, too, but manageable.