One amp - two sets of speakers

I hope this is a simple problem.  I have one amp (Rotel 930 AX) which supports two sets of speakers.  The manual says if running two sets of speakers simultaneously they must have impedence of a minimum of 8 ohms.

The problem is the second set of speakers I inherited are in the ceiling - they are monitor audio and have impedence 6 ohms.

Will this damage the amp?

Is there a quick fix?

Is there an amp which will handle this for say £150?

Is it better to change the ceiling speakers?

Any advice is welcome.