for the original question, the actual samples had small improvements compared to the bigger step to NX but the quality from doubling of wave rom and especially the large improvement in the built in speakers (with bass woofer) made a significant difference in playing impression.

For the HX6 there were 4 audio outputs wired on the 26 pole connector, which had a screen programming the accomp or sequencer tracks to be routed to each in the later firmwares. However you needed the connection cable from Keysoft (or a pin out to make your own?) and I can't remember if the cable was ever made available (perhaps in Germany?), or if development stopped when the 7k came out, so I never received one for testing.