Hi Jez

You are correct the Yamaha SA2 voices are similar to the articulations that were available in VST instruments about 7 or 8 years ago, (Modern VST can have articulations in double figures) however as they take up a lot of memory (They are effectively switch layer multi-samples) hardware boards have only recently had enough processing power and memory to do simple articulation. (SA2 added an additional sample layer over SA)

Most instruments now though are moving towards sound modelling, where the basics are a virtual representation of the instrument (Piano Strings, Hammers etc.) and then mathematical algorithms are used to process the sound produced exactly like a real instrument. (A sample by its very nature is just a single snippet in time, thus you need a lot of samples to fool the ear into thinking it is hearing a real instrument hence most VST sample pianos are 2,3 or even 4GB in size)

Hybrids are also available which sample the basics of the instrument and then use mathematical algorithms to modify the samples to make them sound real.

The above is why I would never buy an arranger/organ that could not use VST on-board.

Here are 3 comparisons

Tyros 4

Sampled

Sample Modelling

Modelled

BTW Tyros 4 came out a year earlier than the usual cycle, so theoretically the S710/910 replacements should be out about this time next year. (Personally I prefer to get new features and sounds by upgrading via software, as it is much cheaper than having to trade in your old board for a new one)

Bill