Maybe asked 100 times, here is 101 
When I use a send track, lets say with a reverb on it. I put a send device in a specific track. This splits my tracksingal and is going to Master via track (ofc) and via sendtrack(reverbed).
But I want to send the signal to sendtrack, want it come back and mix with my original track and then go to master. Not seperately mixing it through reverb to master:
How can I route it back from send track to my track?
That’s not the typical routing for a send track. The whole point of a send is that it creates a 2nd route to the master.
Think of it this way: multiple tracks can send audio to the same send track. If the sound from the send was then routed back to all the tracks where the sound originated, then all of them would get a melange of the sounds that were sent from all the tracks. Messy.
For your usage the way to handle it is to insert the fx onto your track itself. You can mess with the wet/dry amounts on fx to get the same sound if that’s what you’re looking for.
To prevent infinite loopback errors and possible crashes, audio feedback to it’s source track is prohibited in all manners. (This includes trying to circle audiostreams between two sendtracks)
You will either have add all your other DSP effects on the same (send)track or toggle the “Keep source” button if you want to post-process the audio behind the send-device (which you can do as well).
The problem is that each track needs it’s own DSP chain instance to pull this off. Imagine the following: if we were able to mix some effect (say, a reverb) into each of the tracks, and having the send track return the audio to the source track (like you want to), all the tracks would “bleed” into each other because they all feed audio into a single reverb device. So, maybe your snare track would suddenly have remnants of a vocal track, etc.
One solution could be to have a unique instance of the entire send-track DSP chain for each of the tracks that is using it - in our imaginary example, it would amount to the same thing as replacing the send-device with a unique reverb device for each and every track that were it. So there is no real benefit from such a feature in Renoise, except perhaps from a workflow perspective.
the main problem i want to solve is:
Lets say i have a kickdrum on track1 and i want a slightly reverb on it. Now i have at least 2 ways:
a,
I add reverb to track1, which manipulates the complete signal and messes all later effects, unless i put reverb at the end of the effectchain.
b,
i use a send track with reverb and keep source, which eleminates the problems of a,
But now it comes…i have a nice lowcut on my kickdrum in track 1. And i want to keep this by all means.
Version a, is no problem, cause i can put the lowcut at the end. But maybe i get some conflicts with other effects in the chain.
Version b, sendtrack with reverb gets a lowcut signal. adds reverb and lowcut is gone at least for the sendtrack signal. I could add the same lowcut on the sendtrack, but if i want that sendtrack on other tracks i will ave all these with a lowcut, which i do not want ofc .