Hey there -  I'm noticing a very considerable difference in level when connecting via the g90 vs. connecting straight to the amp.   This is detrimentally increasing the noise floor and causing a great deal of squealing and feedback through the amp. 
  
I've moved things around the signal chain and tried alternatives with gain structure and am confident this is not the issue - the only variable that seems to change output level is the line 6 g90...   
  
  
FYI: 
  
I'm not currently utilising the 'cable tone' setting on the g90. 
  
Signal flow is -  Guitar> 
Line 6 G90> 
Line 6 Pod HD500x> 
Amp>