Hi,
The solid-state amp's impedance is not fixed at 8 ohms or 4 ohms. Think of the amp's internal impedance as essentially 0 ohms (it's typically well under 1 ohm). It's the speaker that determines the impedance (it's an electro-magnetic motor that the amp has to supply power to).
But many modern A/V receivers do not have robust power supplies and heavy-duty heat sinks (for cooling), which would be too costly to include on a modestly-priced 5-channel receiver. Instead, these receivers have switches to optimize the power transfer to 8-ohm or 4-ohm speakers. If your amp lacks those switches and the owner's manual suggests that you not run 4-ohm speakers, then you likely shoudn't because it may trigger the protection circuitry.
However, your 6-ohm speaker is close enough to 8 ohms that the amp will work properly, so don't worry about it.
Regards,
