Hi Jostein!
I don't have much personal experience with SW synths, since a lot of the
easy knob-twiddling kind are graphical only. That said though, I've heard a
few just asthey are. AMS struck me at being very good, though being very good
at Moog not Roland. :-) I've also heard some old Rolands, though I was never
really fond of them. Still Yoshimi is very good for pads, the best I've heard
on Linux.
Otherwise you might try something like PD and build yourself an instrument,
that reflects the features of a JP-8x. I don't know,if you can then also save
patches for that specific synth, since for PureData - or other SW synths of
that type - your whole synth-model will be a patch for a start.
Something, which hasn't been mentioned yet: How about VST-plugins? I'm sure,
there should be something - of not more than one something - to home in on
your specs?
www.kvraudio.com is usually the place to look for those.
As for the knob twiddling: Did you ever consider one of those MIDI
slider/knob boxes (Beringer BCR2000 or BCF2000, Korg Kaoscontrol or what3ever
it was called). They are quite good, if you like to have real controls under
yours hands, while building sounds.
I hope, that something of this might help.
Warm regards
Julien
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