FWIW i have a similar issue with my MK5, except it's definitely the B string. I tried a set a optima golds on this bass. while i like the look, the sound didn't knock me out. and i immediately noticed that the B string is noticeably louder and out of balance with the rest of the strings.
i'm not sure what the technical term for this is, but the wrap on the B string doesn't go all the way to the ball end like the rotosound strings that i normally use do. As you can see from the picture, the B string sits closer to the pickup than the rest do. The only way out that i can see is to raise that end of the bridge to accommodate the height difference. or do something funky to the saddle, which i am not inclined to do.
so, while these strings were an interesting experiment, the way the wrap on the B string is pretty much the nail in the coffin as far as i'm concerned and as soon as they're due to be changed, it's back to rotosounds for this bass.
but this may work for you - it will definitely lower the B string height, maybe you can get it work better for you than it did for me.
if you were in the states, i'd mail you the optima strings and you could try it. they were about $110 USD from juststrings, btw.
your actual results may vary...