Solo gauge bass strings
You may have noticed that some bass strings are offered in "solo gauge" or "solo tuning" as an option. If you're not sure what that's supposed to mean, then read on to find out why they exist, and why someone might use them (which are, in my experience, often two different things!)
What they are: These strings vary only slightly from "orchestra gauge", or "orchestra tuning" which is the normal string for double bass. Upright bass tuning is normally G-D-A-E (high to low), while "solo tuning" is one full step higher: A-E-B-F# (high to low), so solo tuning strings are gauged for that slightly higher pitch, while maintaining the same familiar string tension and balance. So the bass plays and feels the same as it does with "regular" strings - but is pitched one whole step higher.
Why they exist: Why would someone choose a solo tuning string set? Simply, tuning up a whole step, using strings specifically for the purpose, is a means to put the higher pitches more "in reach" for someone playing solo repertoire. It can also provide a brighter tone, and better projection, for a solo performer who is playing melodic lines -- which often tend to use higher notes. There's a long history of solo string performers using altered and raised tunings for this purpose. The string sets that are designed and wound specifically for the purpose allow a player to tune to a higher pitch without putting unwanted extra stress on the instrument (tuning up "standard" strings would make them very tense, and add a lot of pressure to your instrument, possibly inhibiting vibration and over-stressing the bass top.)
Why most of my customers seem to use them: So you just discovered the legitimate, intended usage of solo strings. However, we find that - at least for our customer base, which leans heavily towards players who mostly play contemporary, bluegrass, or jazz styles - there's another popular reason to get solo tuning strings. We find that many of our customers install a solo set and tune it down to "normal" G-D-A-E tuning. Why? Tuning them a step below their intended pitch provides a lower tension than orchestra/medium gauge, to provide a "light" tension option for a string that the player likes. This most often comes into play with strings that offer a solo set, but not a light (aka weich) set; Pirastro Obligatos and Thomastik Superflexibles are two common, popular examples.
Where a stringmaker offers BOTH light and solo strings (like Thomastik Spirocores, Pirastro Evah Pirazzi), you'll usually find that the solo set - tuned to standard pitch - is slightly lighter than the light/weich set, but this isn't a hard and fast rule.
If that second reason to get solo strings sounds more like you, and you want to get a rough, non-scientific idea as to the difference in tension, at least in terms of your current strings, it's easy to do. Just tune your current (medium) strings down one full step, to F-C-G-D (high to low). Play on those a bit; see how they sound and feel. If you like it, and those strings are offered in solo gauge, that's roughly what your bass will play like with solo strings downtuned to E-A-D-G. Simple!