The new waves have nothing to do with older XP/JV except the names and philosophy. XV series has new synth-engine using not only new software algorithms and FX routes but also new A/D converters, DSP units, and true stereo multisamples at a tone level.
So even old (named) patches are more crisp, better sampled and with excellent S/N ratio.
Don't be fooled by old names. The XV sound is the best (not only in my opinion).
I have a K2600 with its excellent piano, a new Yamaha S80 + PLG cards and Korg Triton. After listening XV88, i ordered one and i'm going to sell my old grand.