http://organix.inque.org/index.php?page=jx-3p-upgrade
Page of Laurens Schilling. Here you will find the most impressive
improvement you can do to a JX3P at the moment: A hardware and
ROM update which enables the parallel use of MIDI and the PG-200
programmer; also data transfer via MIDI now is possible. Go there and
get it!
http://www.paintingwithsound.co.nz/upg.htm The Painting With Sound Universal Programmer. A very cool hardware programmer for vintage synths - especially for Roland JX- and MKS-series. At the moment (2010) the programmer is less expensive (incl. shipping!) than a PG200 on ebay.
http://www.paintingwithsound.co.nz/JX3P.htm The Painting with Sounds JX-3P Upgrade. This one will be the killer. It is a complete rewritten operating system for the JX3P including more modulation sources (additional envelopes and LFOs) and additional sequencer and arpeggiator functions.
http://www.sonicware.de/jx3p/jx3p.html Page of Oliver Dahlmann, who planned the same as Laurens Schilling. Olivers page contains a lot of info on the OS.
http://analog.no/cms/mks-30-midi-upgrade
Not the JX3P but the best for the MKS-30. Most infos come from
Johannes Hausensteiner in Austria and were compiled and published by
the guys of analog.no
http://www.bernhard-doering.de/Synthi-Museum/Museums-Guide/Roland/JX_3_P/jx_3_p.html
This is one out of many "The JX3P is a such-and-such synth with ...
features". While all the others are not linked here, I picked this one
because it is a little bit special for me: it shows the advertisment
which made my first interest and the specsheet which made me decide to
buy the JX3P.