@laxu
You can also work out your reamp level without the need for a multimeter by running the reamp signal into your DI with input gain at minimum (10dBu of headroom) but its a bit more fiddly.
You can run a sine wave at 0dBFS (i.e. maximum level) from your output, run that into your input chain. If that is clipping, then your reamp level is higher than 10dBu - from there you could keep lowering the sine wave output level until it gets to -0.1dBFS on the input level, and then add whatever level in dB you had to reduce the output by to 10dBu to get the reamp level.
If your reamp level at 0dBFS is below clipping when coming back in to your instrument input, it means your reamp signal is less than 10dBu. So whatever level it is below 0dBFS, take that from 10dBu (so if it comes back in at -7dBFS, it means your reamp level would be 3dBu).
You can also set your reamp send to match your 10dBu input level by adjusting the knob on the reamp box so your reamp level going out matches the DI level going in.