The D-70 and the U-20 have almost the same sounds, similar sound architecture....they both used the SN-U110 cards, but the JV-1080 has a totally more advanced and incompatible sound chip.
The only thing you can do is find a similar PCM wave on the 1080 that sounds like the PCM wave used on the D70/U20 for that particular sax patch. Turn off all the FX on the 1080 except maybe the chorus and reverb as that is what the D70/U20 had. Then make a new patch. I'm sure you can find a very close match. -
Try the Gerneral Midi sound set on the 1080, look at the saxes, they tend to sound more like the old PCM sounds.
I hope you can understand that, it's the only way - for as far as I know Roland would probably not have recycled those older sax waves as they are pretty bad, but retro is in these days.....
I once had a D-70 and a JV-1080!