I was intending to post a query as to why the JD 990 holds its price over the 'improved' JV 1080?
It depends on your interpretation of 'improved'. A little history.
1. JD800 is the first of the 'JD/JV' engined machines. Personally, I adore it - it has a full bottom end when needed but the brightness and sparkle at the top. It sounds like a digital synth which puts people off. Also, the 'yoof' announce that, despite demanding a synth like the old analogues, the front panel is 'too complicated'. JD800 is a commercial failure.
2. JV80 and 880 (and the JV30 in a scaled down form) is the response - 8 sliders only so you can tweak away at the filters, etc (JV30 hard wires it's 3 sliders to the filters). This also uses the new JV engine and has the first expansion slot. It has strings, dance piano, etc. It sells. Roland happy.
3. JD990 comes out. It also has the expansion slot and can read the patches (indirectly - you have to load them into memory) from the Vintage board. The JD engine is tweaked to add a couple of bells/whistles (sync,etc). Basically, it's a programmers machine which sounds even more wonderful. Guess what ... it doesn't sell ...
4. JV1080 comes out. Plus other JV keyboards. Now Roland are messing with the system to try and make it more appealing. One 'improvement' is to add a bundle of expansion slots and allow direct access to patches. Another 'improvement' is to tweak the output to give it a slightly hyped bass end. And it finally has a decent amount of polyphony. It's not as shiny and bright / full as the JD engine, but kids love em and buy bucketloads. Roland back port the JD990 display onto it and call it the JV2080. Kids now buy that. Roland Happy.
The important thing here is that the JD engine was full 44.1kHz sample rate waveforms and the JV was 32kHz which accounted for some of the clarity. Interestingly, the JV80/880 sound quite a bit better than the later machines - more like the JD series to my ears. But the 1080 had a ton of usable patches and could have a load of cards. Couple this with the generous polyphony and it was a great work horse. My only problem was that I never really got on with the sound.
It's worth noting that the XV series uses both sample rates and sound a lot better IMHO.
So why do people rate the 990 over the 1080? One answer is scarcity. The 1080 is ten a penny - there's loads of units out there, but less of the 990s. Another is that the 990 is bought by people who know just how great it sounds and want _that_ sound rather than just 'some sounds'. And one more answer might be that it's a direct replacement for the JD800 - and people are getting nervous about lugging one around these days - much better to get the (thoroughly excellent) rack equivalent..