Stereo cab panning to center to combine two IRs

[Chiguete]

When I try to combine two IRs using a stereo cab and panning them to center I hear the tone get darker/lower... is this normal? Should I add a couple of dB in the level ok the block? If so, is there a specific value to add in the level?

Or is it just that combining 2 IRs in a stereo cab just doesn't work the same as if you would do it on Cab-Lab?