Well, @Androxylo , your case certainly highlights how despite cleaning our mains supply to the nth degree we can still of course encounter noise problems further downstream - often via cable construction, but also from simply equipment positioning relative to each other...(possibly the latter in your case?). Re. the former, certainly some kind of shielding is always recommended, especially with DIY fare...(congrats on your XLR cable by the way - shame about the non-shielding lol! 
). For years now - even if just as insurance - I construct ALL my cables (power and signal, including digital) with separate for pos/neg; signal/return(gnd), which often minimises the need for additional shielding in fact (the resultant lowering of inductance and conductance already providing good suppression of EMI/RFI). But further shielding is the icing on the cake 
.
Hopefully you have managed to tame your noise more consistently now.
While here, I think this might be a good juncture at which to reiterate the difference between dealing with "noise" (at any frequency) that is audible and the possibly less obvious kind that can have negative impacts upon final sound quality - especially in highly resolving systems. Once again, this is where I personally believe balancing the mains power supply (non-intrusive) is far preferable to the 'intruding' approach used in filtering/conditioning...especially given that some folks doing the latter still actually recommend plugging an amp direct into the wall, which may well improve dynamic delivery but leaves it at the mercy of said more subtle mains saturated "noise"!!
... I rest my case...HAPPY LISTENING y'all...
). For years now - even if just as insurance - I construct ALL my cables (power and signal, including digital) with separate for pos/neg; signal/return(gnd), which often minimises the need for additional shielding in fact (the resultant lowering of inductance and conductance already providing good suppression of EMI/RFI). But further shielding is the icing on the cake .Hopefully you have managed to tame your noise more consistently now.
While here, I think this might be a good juncture at which to reiterate the difference between dealing with "noise" (at any frequency) that is audible and the possibly less obvious kind that can have negative impacts upon final sound quality - especially in highly resolving systems. Once again, this is where I personally believe balancing the mains power supply (non-intrusive) is far preferable to the 'intruding' approach used in filtering/conditioning...especially given that some folks doing the latter still actually recommend plugging an amp direct into the wall, which may well improve dynamic delivery but leaves it at the mercy of said more subtle mains saturated "noise"!!
... I rest my case...HAPPY LISTENING y'all...
...Glad you're on course at least...