Hi,
This is all just numbers. Use your ears! Listen to the unit with headphones at your dealer and listen hard for the "grunge" sound noise decay. The samples may be stored in 16-bit but so are samples in all samplers today. There are some very good sounding samplers.
There is a lot going on inside the box. When the raw samples go through the filters, envelopes and even the internal fx, there are plenty areas for the sound to deteriorate. Typically, in these processes, the samples are being processed with a higher bit depth to avoid truncation on the calculations. Then, the sound must be dithered down to whatever the D/A bit resolution is (16-bit or 18-bit in this case) before being output through the analog outs.
Okay, that was more numbers. Good or bad sound is not only dependent on the D/A converters. My point is more, don't look at specs or numbers. If it sounds good, it sounds good. :-)
HTH
Fernando