Manley Steelhead - using the VARIable output

I am currently using the FIXED output on my Steelhead, running into an integrated amp (Ayre AX-7e) as a phono-pre only.

I want to try the VARIable output so I can use the SUM (mono) switch for my mono recordings.

Can I run the VARIable output into an integrated amp? This may be obvious but I am not that technical to understand if this could cause damage or will just flat out not work.

It will work,provided you don't overload the input of the integrated.Check the integrateds' overload spec and listen for distortion.I would keep the volume attenuator very low.This is akin to driving a preamp into an integrated,actually this is what you are doing.
What you would probably want to do is try it 2 ways. One w the attenuator wide open (or actually at unity gain if you can tell what that is) on the Manley and use the Ayre to control volume; and then vice versa. Use the method that sounds best.