I think I've solved my Ibby Mezzo 5 string low B problem.
As I've posted about before it comes with D'Addarios but D'Addario don't make a medium scale string for public sale, they only supply them to Ibby for this bass. I've tried a load of different strings. When using flats I use Dunlop medium scale flats. They have recently had price increases of nearly 100% so I started to look for something else and I thought about going back to rounds.
My current roundwould string of choice are DR Hi Beams but they don't do a medium scale, and for the B string tuner post placement a long scale B won't work / fit on the mini tuners.
So I've now got a hybrid set. E-G are DR Hi Beams 45-105, and the B string is a custom length Newtone 0.136 I tried the Roundcore Platinum option to get as close to the Hi-Beams as possible. The Platinums are advertised as having a smaller wrap wire for lower finger noise and that they have a more broken in feel. I also went with a tapered option.
Tonally they are very close - in fact closer than some other single make sets where the B string seems to be just a random addition. The 0.136 also is quite balanced in tension to the E and A of the Hi Beam set too and I didn't have to widen the nut slot at all. I expect they'll sound even more like a single set once they've all been played in and dulled down a bit.
I'm very happy with it, and the cost is very reasonable too.