low xlr output

I'll guess you are bypassing the pre (direct mode). 

An XLR cable has an in phase signal and a 180 out of phase signal when it arrives at the pre.  That's why there are two 'hots"  in the cable.  If you are bypassing the pre (direct mode) and using the volume controls of the players instead, the XLR signal is probably bypassing the diff amp in the pre.  

The diff amp amplifies the difference between the two hots.  When a 1 volt peak signal arrives  at +1 volt, the other arrives at - 1 volt.  The diff amp "sees" the difference (2 volts) and amplifies it. That's where the boost in volume (gain) comes from.  Any EMF picked up by the XLR cable is  in phase (same in each hot conductor) so is eliminated at the diff amp, which only sees a difference.  

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Just to clarify, low signal strength might be referred to as weaker, not quieter, as quieter means less noise

weaker signal should not result from using XLR in my experience.

quieter relates to ’noise’ in the signal, thus quieter is actually higher signal to noise ratio, which many believe is the advantage of XLR, which primarily exists for pro use to prevent interference in long runs of cables, crissing, crossing, adjacent to motors, amps, space heaters, the like.

yes weaker sound.. so eg volume with rca at 25 is same loudness as xlr on 30

there are some jumpers inside the unit close to the xlr connections

any idea what they are for ?

photo attached

Not about the jumpers but...

From Stereophile:  

"The maximum voltage gain with the R200i's volume control set to its maximum (an indicated "59") was 46.5dB for both the balanced and unbalanced inputs."

Could this be a clue?

Not Sure