Please save me from an expensive mistake! Sonus Faber Amati Futura longevity concerns...

Fair points.  I wasn’t actually suggesting that you buy spares for such an expensive and complicated speaker. ;-)  You say you love the sound, the woofers have been replaced so they didn’t seem to spoil what you like about the speaker, and they should last for a long time.  That leaves the tweeters.  I understand your concern but I would ask the dealer if they can or should be serviced, and how much that would cost down the road.  OTOH, if this is always going to be a worry, like with one commenter above, then stop right now and move on.

Something that does bother me a bit, however, is when you say that the other SF and the Spendor 100 were "bass light" in your small room.  Perhaps some treatemnt woukld help, but I can't help wondering if you're buying too much speaker for the room.

Unfortunately past experiences have made it clear to me that the dealer will not always keep his word.

Please correct me if I'm wrong but "too much speaker for a room" usually results in uneven, excessive and uncontrolled bass, not in an overall feeling of lack of bass. Maybe I'm placing the speakers in a null, but that's where the midrange and highs sound the best, about 1,2m out in the room (measured from the front baffle to the back wall) on the long wall.

Somebody told me that  "you have to check if the tweeter you want to repair is easy to open. Some tweeters have a ‘click system’, or are glued together, and therefore barely serviceable" . Anyone knows if the tweeters in the Futura are serviceable?

Not sure where I've got this information from a while ago, I might be wrong, but the tweeter might be a (modified?) Scan-Speak Classic D2905/970000 while the midrange driver might be a modified Scanspeak 18W8545. FWIW

@donquichotte - @knotscott : You are saying that  ferrofluid replacement should be"Fairly easy to do if the tweeters come out, and the voice coil assembly is removable." How can I assess if this is actually the case with the Futura's tweeters?

I'd ask the dealer and/or Sonus Faber

By "too much speaker" I mean that the speakers don’t have the space to open up, and thus are restricted from performing optimally.  Not to put too fine a point on it, but I’’ve seen it often.  A good friend with good ears had a pair of huge Focals in a very small basement listening room.  They never could do what they were designed to do and eventually he got rid of them.  Ditto the Spendor 100s.  I’ve heard them in a small apartment and they just couldn’t do what they were supposed to do.  How big is your room?  If it is less than 20 x 12 feet, I would strongly recommend a much smaller speaker.  My living room is 20 x 12 and the Response 2’s do well but some Tablette’s or Reponse 1sc’s would probably provide a more realistic soundstage.