Well I more or less wanted to make sure I got a good feel for the S5's before I posted any more impressions. I knew I needed to experiment with placement and I did want to go back to my Alexias to confirm the differences that I thought I was hearing. I'm stil playing with speaker placement to some degree and I haven't went back to my Alexias. I had a friend help me move them out of the room and I can't move them back on my own. I keep putting it off. If they were still in the room I would have gone back a while ago.
What I feel I have learned is that both speakers share some common ground. Both are excellent products and capable of great sound when setup correctly. Both have their advantages. Both allow me to hear the differences between electronics, cables and power related gear. The Alexias have a bigger scale and a stronger sense of power. The S5's are more restrained. Both can be fatiguing when not setup well. I prefer a sub with the S5's (even though I have two, I've only hooked up one so far).
With both speakers I hear some artifacts that are identical - that tell me the issue is not with the speakers themselves, but somewhere else in the chain, setup, or the room itself. I feel I hear less artifacts with the S5's. I feel the S5's are little more neutral, perhaps a little more clinical. I often turn the volume up much louder with the S5's to try and manufacture some presence and scale. I wish there were larger Magicos in the S series.
There are some intruments that I prefer on the S5's - like classical guitar, clarinet, orchestral cymbals & vocals. Rock sounds more 'live' and in-the-room with the Alexias. I'm not thrilled with violin on either speaker (which probably says something about either my expectations or a problem elsewhere in my system). Black Sabbath on the Alexias rule.
You may not be able to fully appreciate the S5 with the Pass you have, it is not be powerful enough for what you are trying to do (there is a reason they go up to Xx 1000 ).
FWIW, I did hear that Martin Collomes ended up buying an S5 for his reference speakers. In some ways, he has been through the same process we have, using Wilson as reference for many years.