I would love to get your thoughts on other amps that work with the S5's.
I have heard Magico with the following amps. Within each category, the amps are classified by order of preference (starting with best). For the amplifier with which I had only short experience with a Magico speaker (around 1hr), I have put them in italic:
Gold standard
Vitus SL102+SS101: amazing, really sweet sound (a tube sound.... with the slam in the bass and the extension of the best solid state amp. Only slight weakness is that highs are very slightly recessed compared to my Dartzeel for example, but this adds to the sweetness). Would work with any Magico speakers. Only the Q5 may further improve (?) with a bit more power- would have to hear the Vitus from the Master series
Dartzeel NHB-458: best amp I have heard with the Vitus combo. Best amp for Q5
Vitus integrated SIA-025: never heard a 25W amp sounding so good, and it sounds like much more than 25W. If you listen very loud, you may want more power, but it is an amazing amp! Works well will S5 or Q3 if your room is not a cathedral. May be a bit underpowered for my Q1 (Q1 has lower sensitivity), but I still have one coming
Dartzeel NHB-108: my current amp. Not as sweet as the Vitus, but very fast, with an amazing decay. Enough power for all Magico speakers except Q5. Balance is a bit thinner and a tad more transparent than the Vitus. Planning to compare it with a Vitus in my room to have same reference.
Gryphon Colosseum: Class A sound, similar to Vitus. May be higher in the list, but didn't hear it long enough to be sure
Silver standard
Hegel combo: excellent sound for the price, great value for money. When showed at Tokyo audio show last year with the S5, it was significantly better than the much more expensive Pass mono amps they were using alternatively
Soulution 700 and 500 series: super low noise floor (which makes it a candidate for my Ultimate), super transparent and neutral. But a bit cold (like the Spectral, but better than a Spectral), I would like something warmer without losing on details
D'Agostino amp
Audio research Ref150: I am not a big fan of AR, but I was positively surprised by this model. Sounds different from the classical AR. Works wonder on Q3
Spectral DM-400: very transparent, very fast, but a bit "mono-tone", lacks richness of tone of my Dartzeel, a bit too cold for me to be my long term amp.
A big Boulder (don't remember the number, but sound was nice- may be higher in ranking)
VAC450 reference: heard it on Q7. Super sweet mid, but lacks extension in the highs vs. the best solid state, and bass is sluggish. Typical tubes sound.
Bronze standard
Constellation Centaur (stereo): average quality of construction (open one, what a mess). Sounds very good, is not doing anything bad but also nothing great, but would expect more at this price point. Not the ultimate transparency. I however still prefer it over the 110V DM360
Devialet D-Premier: excellent solution for a no thrill system (have also one). One box for streamer, DAC, pre and power amp, so very good value for money. But sound quality not at the level of a solution with separate elements, or of a Vitus integrated. With Q1, sounds improve significantly when you use 2 in mono configuration
Spectral DM360: had mixed experience. on a 110V model, very fast amp but cold, and bass was sluggish (worse than the Constellation which I listened on same day, with Q7). On a 220V model, same characteristic but with outstanding bass. As often, 220V version of amp works much better. And I had the impression that DM400 was a significant improvement (but I listened to them on a different day)
Ayre MX-R: nice monoblocks. Sounds quality at similar level as the Devialet.
Not a good match
APL hifi UA-S4: I have one (hybrid tube-solid state) that I was using with my APL speakers. Lacks current to work well on Magico.
McIntosh: Believe it was the Mc452 or Mc302. Sluggish, slow, lacks transparency. Like all solid state mcIntosh I heard in the past....
Not heard but probably competing for the Gold standard category
MSB M203: heard very good things on it from 2 trusted Magico owners, I have never heard it myself
Gryphon Mephisto: Jeff Fritz is raving about it driving his Q7. Have not heard it yet, but if it is really better than a pair of Collosseum then it really must be something!
On specific fit of an amp on different models of Magico: All Magico speakers are very transparent and neutral. If an amp is good, you will hear it. If it is a poor design, you will hear it. The only caveat is that some Magico are easier to drive. Q5 is the most demanding: works well with a lot of power, and a lot of current. S5 is in fact not so difficult to drive. Easier than a 2 way like the Q1. But for what it is worth, my 150W Dartzeel never clipped on the Q1.
Q3 and Q7 are the easiest to drive. Any amp on the list above would work well.
On S5 vs Alexia: with S5 you have more bass extension, cleaner bass (when your ear get used to closed bass, you will find any bass reflex bloated and slow), more transparent mid range and sweeter tweeter. Speaker overall is also more neutral due to more linear frequency response. I have moved away from Wilson a while back and did never look back. Even if an S5 is 40% cheaper, I would take an S5 over an Alexia.
PS: on using variable resistors to adjust tweeter on your Alexia: it introduces impedance issue, which can affect phase. It also changes frequency response of tweeter which (in passive XO) can affect balance of all of the XO. Not a very good idea, flexibility of set up has a cost.
Hope it helps!