I recently migrated from the Juno DS to the FA-06, a world of difference in sound quality!
Well, I read you sent it back, did you switch between SN-A and SN-S modes? When I received my Fa-06 I thought it had horrible sounds in it but when I switched between those modes I confirmed I made a great keyboard choice, imho. For those who just unboxed their FA series keyboard this is the setting you need to edit:
The FA has three distinct kinds of sounds (besides the drums):
* SuperNATUAL Acoustic (SN-A). There are relatively few sounds in this category, but it is where you would expect to find the best patches for these particular sounds: piano, tine and reed EPs, clav, organ, acoustic and electric bass, acoustic guitar, and ensemble strings. These are "Integra 7" quality, and should generally beat the Juno DS equivalents.
* SuperNATURAL Synth (SN-S). These are the sounds that use their modeled "virtual analog" engine, which is designed for "synthy" sounds rather than emulations of acoustic instruments. This, too, is "Integra-7" tech, and should generally beat Juno DS.
* PCM. These are all the other sounds, and they are all sample-based, regardless of whether they are "synthy" sounds or other acoustic instrument emulations. These are derived from the old XV-5080. When it comes to acoustic instrument emulations, except for the particular sounds listed above in the SN-A section, these sounds will generally lag those of the Juno DS, whose sound don't come from the XV-5080 but instead come from later Fantoms and other newer sources.
The are also additional PCM-based sounds available on the SRX-based expansions, which can be loaded into either board (one set into the DS, 2 sets into the FA; but the DS also allows you to load your own sample-based fully playable sounds into expansion memory, whereas the FA does not).
So there really isn't a straight answer to which of these boards one would expect to sound better... it depends on the particular sounds you're looking at.