Headphone cue amp distribution problem

So i've added an extra cable run from power amp to supply extra rooms.
One cable run's approx 20metres [18SWG/1mm2] and other (new) is maybe 40metres. [20SWG/0.75mm2]
Both cable runs start at the amp.

Individually both fine, but if i run both together i'm getting some hum on the cans and the amp heats up, alot...

No shorts, wiring is correct.
Headphones are 600ohm (DT770)

What would cause this? 
Is it because of the long cable lengths affecting the output impedance of amp?

Amp's a 30watt RMS into 8ohm Canford/Audionics utility amp. Used them for years, robust and easy to fix when they blow!
This one's boggling me..
thanks!

HF oscillation can cause that symptom...

JR

Long cable runs = lots of capacitance, which class-(A)B amps don't necessarily like, and can push them into oscillation.

Ah Ok.
Anything i can do to mitigate that?

Suppose i'll just have to use separate amps.

Put 10 Ohms 10 Watts in series with each cable.

It is NFB amps (not so much AB amps) which freak-out when load impedance falls to "zero" above the audio band.

+1 what PRR wrote.
10 ohms, 10 watt, in series.
exactly what was done 40+ years ago in studios before the musicians insisted on their own cue mixers.

does the Canford amplifier have a Zobel on the output AND a series inductor ?

Thanks chaps.
Will give it a go.

I actually blew it up last night.. Whoops. Have spares tho.

Works fine now.
I put 4R7 50W resistors in as thats what i had handy.

Amps happy down to 4ohm on outputs.
As a thanks, i attach gut shots.