Hello @ClaB , 

Thank you for your valuable feedback . Please make sure Ethernet cables supports1000Mbps (cat5e or above).

Can you test AV1000 KIT and AV600 separately at the same location and  help confirm some details to locate the issue: 

1. the photo of the powerline rate beween AV1000 KIT on tpPLC APP or tpPLC Utility  (unplug AV600 when test AV1000)

the powerline rate between AV600 KIT when test in the same wall sockets

2.  speedtest on AV1000 extender vs speed test on AV600 extender on the same devices

3.  wired link speed on PC when connects to AV1000 vs AV6000 

 https://www.tp-link.com/support/faq/2899/

May I know the model of router, and your netwokt map, eg : Router Archer C7 <Ethernet> TL-PA7017P downstairs _____ PA7017P upstairs<Ethernet>Windows PC

Thank you very much~