The Pro iESL has regular speaker connections. You only need a speakon to banana plug cable to connect it. It has loudspeaker input and output, because if the Pro iESL is powered off, the signal will be pass-through to your passive speakers (if you have any). That's why I connected not only the input, because I use with my power amp my ELAC speakers.
iFi Audio recommends 25W/4Ohm to 100W/8Ohm power input to drive the Pro iESL. My power amp has 150W/8Ohm so a way more than recommend, but this is not an issue, just careful use of volume to not blow your head off.