I just bought a Defy 7 amp from a fellow Audiogoner. He had the amp set for a 4 ohm impedance to match his speakers. I am using a pair of Definitive Technology 7000SC speakers which are rated at 8 ohms. Before I go and change the impedance of the Defy 7 to 8 ohms, I was wondering if other people have tried the different tap settings and have any thoughts on the issue. The amp sounds wonderful as is and I know that some people even recommend using the 4 ohm taps on tube amps even if the speakers are nominally rated at 8 ohms.
FWIW, as I do not have the Jadis Defy 7, but I have just recently delt with the exact issues you are facing.

I have speakers with a nominal 8ohm impedence, a minimum about 5ohm, and a effiency of 92db. I've become accoustomed to driving them, and all of my speakers for that matter, off the 4ohm taps of various amps, over the years. Just sounded 'better'. Warmer, perhaps but sufficiently detailed/resolved.

Well I have a recently acquired amp which I thought was challenged 'resolution wise' and although I enjoyed it, I missed the 'inner' clarity I love. Just out of curiosity I connected my speakers to the 8ohm taps. I was amazed, it was almost as if I were using a different amp! Mid-range resolution/clairty was greatly enhanced, unfortunately so much so that it challenged some of my sources which were otherwise very enjoyable. The tone was much the same, but the 'speed' was noticably increased and a less than excellent source was readily apparent. Too bad the amp wouldn't allow me to change taps on the fly so I could choose depending on the source (software)!

Bottom line, I think for you, is there is no way you can anticipate without actually listening to the changes that the difference taps will bring. It is totally equipment/synergy and personal sonic preferences dependent. Oh, BTW, I didn't find the change on other amps to be as significant, but these amps never sounded deficient in clarity in the first place, or at least so I noticed.