I tried shorting the inputs and find that I get more hum and more hiss than when there is no input cable connected but this is at quite high volumes.
It should be done with a short wire, which should not be touched.
When I connect up to the TT and cartridge the hiss remains low but the hum increases. This is now almost OK but I would like to get rid of this last hum. I have also tried the Cornet in a completely different system with similar results.
The increased hum is out of scope of the Cornet - check either turntables or interconnects for that. Is it possible to check one channel at a time, just to be shure the internal wirings are OK? (I mean connect only one interconnect's RCA to one of the inputs of the Cornet, listen to, disconnect, connect another channel, listen to...)
With regard to the input load resistance any idea what sort of values should be used?
I think the value to start with can be taken from the cartridge's datasheet. As far as I know Cornet has two pairs of spare holes just for that, where you can connect a resistor to try (without unsoldering the initial resistor). This tuning if only for sound, it is unlikely the loading resistor will help you with fighting the hum and hiss.
I also noticed that holding my hand close to V200 increases hum and more so when I touch it.
Well, actually this is OK. That is why the essential part of eliminating hum is shielding: you should be sure all metal parts of the case are connected to the safety ground.