A non 5 pin screw locking DIN configured so the fingers cant make contact with any live terminals might be ok ,
But the usual plastic Din plug should never be used for high voltage ,
Likewise cable strain relief , its worthless in the standard plastic/metal shell DIN plugs ,
not something you want with high voltages .
The clearances for high voltages between the pins of a DIN are probably a bit marginal by todays standards , probably fine for a tube mic , but for 300vdc , its a bit close.
Extra heatshrink around the solder points on the sockets and plugs is always a good thing , it acts like a strain relief of the individual conductors if the cables are ever moved around .
There was a question about clearances on HV traces on PCB's in another thread ,
I remember asking a buddy about a rotary mains switch I was using and if the clearances were upto modern spec , Its was a three position double wafer switch , so off, on(standby) and on(HT),
There was something like 2mm clearance between adjacent terminals on the switch with either mains voltage or AC straight off the HT winding ,
In the end I went with it despite the low clearance , its worked flawlessly for 20 years ,
