Just my 2c, please don't forget there's different strokes for different folks. Like with everything else, the truth with IEMs is that there's no universal truth. There are detail freaks who value the resolution and separation of multi-BAs above everything else. There are holistic listeners who don't need the last bit of micro-detail and think DDs just sound more natural. Weigh their opinions, see where they're coming from and try to find out who hears things similarly to yourself.
The W4 you've heard are probably the universals that sound closest to high-end BA customs, so you should be very sceptic about going for these if the W4 aren't your cup of tea. Been there, done that and let me tell you, some fundamental characteristics of BAs just don't change, even in a custom shell. You can get deep, fast and well defined bass with high impact, but not the same kind of texture that DDs can provide. Same goes for soundstage, you just won't get the soundstage of open-back DDs from closed IEMs, no matter if they're universals or customs. High end multi-BAs have their undeniable merits, but I'd be highly sceptic about anyone who tells you that they're superior in every aspect. Ask about their preferences among universals and go from there. Trust your own ears, no matter what you're reading on the forums. Good luck for your quest. 
