That was close to what what I was going to suggest.
A heavier gauge.
Velkominn til TB!
I don't mean to rag on you at all here, but may I ask why
you feel the need to detune 2 whole steps to begin with?
For on thing, your cab doesn't produce a lot of SPL at a
low G fundamental, most of what you are hearing is the
first harmonic anyway. And setuning that far does create
the issues you are experiencing, floppy strings, lower tension
requiring higher string action to avoid fret buzz. There are a
lot of reasons to use the 5 the way it was designed to work.
So I am curious to hear what your thoughts are on the
matter.
And another hearty welcome to a Northern bass playing bro!
Edit: Google around for info on Stew McKinsey, subcontrabassist.
Wonderful guy and a proponent of sub contrabass techniques,
he might have some valuable tips for you. He is very accessible,
friendly and helpful.
You will find him here and on Bassnutz at times.