Simple! You can eliminate the noise floor increasing the level of the Boost (floorboard parameters)
at minimum of 13db or higher (depending on the other effects setting)
Attention, high db levels can lead distortion at the send effect!!!
If you don't want to waste the boost functionality, you can choose the stereo compression effect after the amp section (tremolo comp eq) like a volume/gainer effect.
Of course you will have to reset all the parameters of the compression working only on the gain level
attack and release (no matter!)
gain: 13db or higher (depending on the other effects setting)
In this way the send loop will have more gain and will improve the signal to noise ratio.
I use this method with my Line 6 m13 and works very very well !!!
The signal is always clean!!!
This is my routing (4 cable method):
Guitar > M13 mono input > M13 mono send > Vetta front input > Vetta stereo send (Y cable) > M13 stereo return > M 13 stereo output
> Vetta stereo return (Y cable)
In this setup I have a wonderful solution
In this mode my Vetta takes the Pitch Glide, Smart Harmony and other great effects of the M-series.
With FBV switchs,expression pedals (pedal 1 and pedal 2 http://line6.com/footcontrollers/fbv.html) and Vetta MIDI OUT to M13 MIDI IN you can control the pedal1 and pedal2 functions of m13 by midi control changes cc1 and cc2 (without using the other two external pedals of m13) , TAP TEMPO (cc64), program change and some looper functions :
Modulation controls Record/Overdub (cc50)
Delay controls Play/Stop (cc28)
Reverb controls Half Speed (cc36)
If you don't want to use Looper functions, you must lower the Looper play level (from M13 setup)