I'll give this a go. These are just my experiences so others may vary obviously depending on taste and gear. I'll talk in a general sense because I've either demo'd the gear or owned them with different components running them at any given time.
DCi - made me a believer, so much that I'll always have some model of 3A throughout this hobby. Excellent with instruments and vocals. When people say lack of listening fatigue, they were right. I could play music for hours and really enjoy it. I found positioning was very difficult but when you hit the mark, it's bliss. I've yet to run it with tubes but from what I hear, it's supposed to be that much better. Overall, a big and loud presentation with control to match.
Dulcets - I passed by these at last year's montreal show and again, really amazing musicality. I found the flow of music was much easier than the little ProAcs (less efficient).
1SC - this is what got me into this hobby. I'm a huge fan of ProAc (1.5, 2.5,etc..). It's remarkable what these small boxes can do. Of all the models, the 1SCs are my favourite. I've heard them in big (23x14) and small (10x10)rooms and they sound great. Very accurate and tight in bass but it won't shake any walls. I found the more wattage you gave them, the better they performed. Some say tubes are the way to go. That's cool too.
Tablette 50 Sig - compared to 1SCs, these couldn't keep up. There's still that wonderful proac sound but it was outmatched in all aspects. Again, just IMO.
Merlin TSM - spent only a little time with these so I can't say too much other than they were pretty nice. I think I'm biased after listening to the 3As. I'm also not crazy about the design or appearance of it. I know it shouldn't matter but it does.
Hope this helps.
