A bit late to the party, but if I take a peep at the specs of ER-1 and Rytm (assuming MKI here?), the problem becomes obvious:

Electribe ER-1 outputs at -10dBu
Rytm MKI input expects a level of up to +15dBu

Therefore, you have a whopping 25dBu (!) mismatch with this pairing… No wonder you need a mixer LOL

People really need to learn to “spec shop”… this problem would have been obvious just from the manuals. There is no way to pair the two nicely without some gain booster between them. Solutions for this sort of problems do exist however, you’ve got boost pedals, line level shifters etc… Mixer is only one way to tackle this

just my 2 cents (jm2c)