1. Onn ONA13AV503 mini-stereo. Its OEM speakers are rated at 4 ohms. I have two Panasonic [M# SB-555] speakers rated at 8 ohms, 12W.
2. Aiwa CX-NAJ20U stereo. Its OEMs are 6 ohms. I have two Sears 2-Way [M# 564.92904551 ?] speakers rated at 8 ohms, 50W.*

*The manual for #2 says the surround-sound speakers can be 8-16 ohms, but I could not hear the vocals when played through there.