I have a Motu M4, and a Focusrite Scarlett 2i2, and they both say the same about line inputs
XLR Line Out to XLR Mic In equals Smoke. Use an XLR to TRS 1/4". My PreSonus has a "Line" button for XLR/TRS combo inputs 1 and 2 to switch the TRS inputs from Instrument to Line level. The XLR In is Mic only. With the amp speakers cut off with headphones plugged in, use the Master volume to set the DI output level to something that suits you.
I agree with Maarti, in general, since the Scarlett User Guide says the XLR Input
automatically uses the internal preamp. (The Motu User Guide is less explanatory with a straightforward "do not" statement, but I assume a similar situation.)
" ... ... The Scarlett 2i2 has no “Mic/line” switch – the Focusrite preamp stage is automatically configured for a microphone when you plug an XLR into the input, and for a line or instrument when you connect a jack plug. ... .... Set the INST switch to OFF if you are connecting a line level source such as a keyboard, synthesiser or the balanced output of an external audio mixer via a 3-pole (TRS) jack. Note the Combo connector accepts both TRS and TS types of jack plug."
However, with all due respect to Maarti, I'm not so sure that "XLR Line Out to XLR Mic In [
automatically] equals Smoke."
If you just
cannot get a good full sound using the TRS Input, and if you feel very adventurous, you
might try the following at your own risk. Keep in mind that this is apparently
not recommended by the manufacturers.
However, since the Rumble XLR Out level
is controlled by the Master control, It
may be possible to use the XLR Input on the Scarlett if you start with a very low, or zero, Rumble Master level. Slowly working your way up to a usable, but still low, Rumble Master level may be the key you'relooking for! You should probably turn down the Scarlett's Input Gain,
and turn down, or off,
the Rumble’s Gain, Overdrive and EQ, at the beginning, to be
sure not to overdrive the signal going to the Scarlett's preamp. Then see if this works OK as you s-l-o-w-l-y turn up the Rumble’s Master for testing while diligently monitoring the Scarlett for distortion and
overheating. If there is no overheating then, go back to a low Rumble Master level setting and try adding EQ to taste for more testing, again listening very closely to the Scarlett's output for distortion, overdrive or clipping, and still monitoring for overheating.
This might possibly give you the fuller sound you're looking for, but please be very, very careful, since this is not recommended by the manufacturer. And, good luck.
What say you other Scarlett and/or Motu users about this?
.