They were reviewed in the March 1989 High Fidelity magazine, but I don't have the issue. It is offered on the web, and you can send an email to this address to ask:firstname.lastname@example.org
I would suspect that they are 8ohm speakers. As for the Onkyo, here is a direct quote from the manual:
"1. Check the impedance of the speakers you are connecting. The TX-DS595 requires speakers with an impedance of 4ohms or greater. Connecting speakers with an impedance of less than 4 ohms may damage the TX-DS595.
2. Set the IMPEDANCE SELECTOR switch according to the impedance of the speakers being connected.
If all speakers have an impedance of 6ohms or greater, slide the IMPEDANCE SELECTOR switch to the left (6 OHMS MIN./ SPEAKER). If one or more speakers have an impedance of less than 6 ohms, slide the IMPEDANCE SELECTOR switch to the right (4 OHMS MIN./SPEAKER)."
This directly indicates it will function with any load ABOVE 4ohms, within reason.