flabby lowB.. experimenting

[Magneto]

Hi forum,

I'm experimenting around, trying to tighten up a flabby action on the lowB on a semi-newly-acquired Peavey 34" scale 5 string.
I've been reading various posts and ideas here.
Not sure what what brand strings were on the bass when I got it, but they were roundwounds, probably 45-125s.
Restrung it with some fresh DR Hi-beams 45-125(my favorite for my other bass), and had the same lowB issues.
Decided to go try a .130 or .135. Only single I could find around town was a D'addario nickel roundwound in a .130..
I installed it thru-body (untapered won't fit the bridge slot), and it made a slight improvement, even though I'm pretty sure it's a hex-core string, where the Hi-beam is a round-core.
Barely fits the nut slot, which concerns me, because I know that a 135 won't fit it, which means I would have to file out the slot for it.
I have read a bit about tapered strings. I guess they are only tapered on the ball side, correct? haha.. excuse my ignorance.

I did raise the action a bit higher on the lowB, and it helps also, but still needs to be tighter to play successfully with even a light touch..
I like this bass alot, but if I can't work out this issue then it's of no use to me...

Thanks for your ideas and suggestions.

Mag...