If there's no sub connection as you say it has + - only, you might be able to use that with your sub if it has them too.
You can connect your speakers fine in my opinion. I don't see an issue but that's also in part because I don't know much about ohms on speakers and amps. What i do know is I've got 4ohm speakers and have had them on 8ohm recovers and never had an issue.
Your receiver might say 6ohm but less can work fine too. Maybe the thing is volume played that is a factor in ohms. So if not blasting then no problem in my opinion.