So you are saying I should look to getting the solo F# 4/4 string and then down tune to a low - B (5th string)
The bass I use, uses 3/4 strings though.. but I should still get the 4/4?
Of course NOT!
You need to be more specific which strings you already use, 3/4 or 4/4 (not bass scale but string type), Solo, Weich, Mittel etc. and if you refer to Solo strings the tuning (i.e. downtuned to orchestra tuning).
If you bought Spiros in the US, they are most probably 4/4 strings. You can only identify them by unshortened length, not by silk color. Or you still have the sleeves and can find S42(W)/S43 for 4/4 or a 4-digit number starting with 3 for a 3/4 string.
There is only a low B/C# for 4/4 Spiros, for the 3/4 there is only a low C.
Best get my excel file from my Dropbox, the link is in the last DB string forum sticky close to the end.
But unless you won't like to buy a whole new set and probably get disappointed because it's a different tension than your last one, try to identify your current set first.
When you have identified it, use the excel file to find a low B for you.
If you need to downtuned a low C, use the Formulas.html to calculate the downtuned tension. It's about 10% less than on original pitch.
The tension difference between 4/4 and 3/4 Spiros or Superflexibles of the same kind (Solo, Weich, Mittel etc.) is about the same amount, 4/4 Spiros or Superflexibles have about 10% less tension on the same scale as 3/4.
But get that data from my Dropbox first and have look at it before answering more questions. And identify your strings, there is some info about the lengths for Spiros in that Dropbox too.