OK, so live and learn - never heard of solo tuning until now. Here's some info that some of you no doubt already know, but I (and maybe others) didn't:
Solo Tuning
The second most popular tuning is called Solo Tuning. This is also a fourths based tuning, but is one step higher than Orchestral Tuning. The result is F# B E A:
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This tuning is most commonly found when the bass is playing a more “up front,” solo role in classical music. For example, when someone is playing a Bass Concerto. Hence the term Solo Tuning.
Classical bass soloists in the 1800’s, such as Giovanni Bottesini, began experimenting with string materials and tunings in an effort to give their instruments more volume, projection and brilliance. They were trying to give the bass a more soloistic timbre. The tuning above took hold and is still used by contemporary players and composers. Although some players fly the anti-solo tuning flag, a vast repertory of works exists for bass with this tuning and contemporary composers still write for it.
In fact, Solo Tuning is common enough that most string makers produce both Solo Gauge sets for each of their string lines. If the strings are listed as Solo Tuning, or Solo Gauge, they are designed for the tuning listed above.
Solo tuning strings tend to be thinner than orchestral strings, making them somewhat easier on the left hand as well. It is not uncommon for Upright Slap players to buy Solo Tuning Strings, and tune them E A D G. This puts the strings at a lower tension, which makes it easier for both the left hand and the slapping hand.