Here's a trick to couple your guitar to your modeller and your power amp properly.

[vino_buono]

I use a Source Audio Zio buffer-preamp after the wah and before the modeler. It works great at shaping the signal (especially the Pultec mode -- great for live). I did not notice any change in "feel", though.

 

[Sedaxel]

The only important thing of an input buffer is to ensure a good impedance coupling between the pickups and the active equipment that it's going to treat the signal, so it doesn't load the pickups and keeps the frequency content healthy. It could be a benefit if the modeller (in this case) has a not-too-good input buffer... But generally speaking, good units have good buffers. I doubt a little about the Helix one, since I didn't like what my Helix did to the signal with everything turned off. But even so, I can't be sure the guilty was the buffer.

Once the signal has entered the active path, the signal is just carried by the electrical power of the equipment. There's no fx-loop "passive buffer" in a modeller because the signal outs from an active system (with its DA converter) and comes back to an active system (the AD converter).

The only thing I can think a loop buffer could do is modify the level (raising it if it's low, or the other way around). If some coloration is involved, instead of level adjustments, then it's acting as another effect in the chain, so you're not keeping the modeller tone intact. It can be great, of course... But it's not the modeller alone.