What do you expect of the Motif that you don't think you can get from your current synths?
If you want synth sounds, I don't think new ROMplers have improved at all in that department...
At least the sound of the Kronos seems pretty similar to the sound of my Triton (considering only the ROMpler section, of course).
On the other hand, if you start talking about "real" instrument sounds... then you can clearly notice a difference, but from my experience, some samplers can do a very good job at those sounds, and you can find them pretty cheap.
My EMU E6400 Ultra is excellent at those kind of sounds (I got it with a nice library) and it was pretty cheap and can be found for nice prices easily.
This way, I don't think I would dump the Korg Triton...
I have one and quite like it and think it has a pretty good sound and you can definitely get great synth sounds out of it.
But the Korg X5D I would dump, unless you need a small and light ROMpler to take with you, despite I think there are better solutions.
I would likely dump the Roland JV2080, as I have had 2 JV1080 and don0t really think they are that good (prefer the Triton sound at least).
But if you like it and use it, just keep it
If you sell one of those you should have enough money for a good sampler with some library that will fit your needs.
I would recommend the EMU E6400 Ultra, as I think it can give you nice realistic sounds.
And if you like the Roland JV2080, I think the Roland S760/S770 could be a good choice for a second sampler, if you'd want a second one.
I would also think about selling the Yamaha DX7s too, since you are thinking about it, for a simple reason... you might be able to sell it for a nice price and you can get a DX7IID later for the same price and that one seems better.
The Korg Prophecy seems to be a keeper, especially since you don't have other monophonic synths.