Ian, many of the new SA sounds share the same sample set. It's not like they are ALL different. Some may indeed simply be re-workings of older sample sets, too. Same for Mega Voices, etc.. You KNOW how the marketing boys like to play the numbers game..!
It's not until someone gets their hands on one and investigates it carefully are we likely to know exactly how many BASIC sounds are completely new.
There are some with T3's that will say ANY improvement is 'significant'. Me, I like to keep the word for something that truly is. Distance tends to be the final judge of this. The week that something new comes out, everybody is giddy with excitement. A couple of years later, maybe not so much! In retrospect, the T1-T2 shift WAS significant, SA opened new doors and upped the ante bigtime. I think most Yamaha users would agree. Would so many of them think the same improvement came between the T2 and T3? Somehow, I doubt it. I certainly didn't get that impression from those that made the jump when the T3 first came out, or considered it after a couple of years of reading posts about it.
Will the T3-T4 jump be as significant as T1-T2? Only time (and lots of it!) will tell, IMO. So far, only the usual Yamaha fanboys seem to be throwing the word 'significant' around. Let's wait a while, and see if less biased listeners join in...
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An arranger is just a tool. What matters is what you build with it..!