Definitely not a problem in the signal chain. I do run a Line6 Helix, but for the purposes of troubleshooting, I ran direct into both amp with multiple different basses using both heads with both cabs and the problem remained.
The studio is nearing 18 months old, not had issues before, but I'm certainly not ruling it out.
I think my first port of call will be to run the amps through a different power outlet, see if that has an impact - if it does, then that's the issue. If not, then maybe re-valve one of them and if that works then I'll do the other.
I suspect now it may a power supply problem...