Lead Tone - screaming feedback

Hey - recently purchased grandmeister 36. Great amp. Just having a problem with the lead tone - I get screaming feedback which, even with careful muting is difficult control. Seems every time my fingers leave the strings the guitar screams and I can only completely stop it by muting with both hands. Tried different guitars so I am sure it's the way I have set this patch up (the values are as shown on the APP) Here are the settings - can anyone suggest what I am doing wrong;

Reverb - 12. Slight delay. No other effects
Presence - 40
Resonance - 67
Noise gate ON
Treble 62
Mid. 54
Bass. 81
Volume 83
Gain 51
Boost ON
Channel LEAD






I am using a n Epiphone LP guitar with stock pickups.