Alsa Modular Synth (AMS) (version 1:2.1.1-1kxstudio1 shown in right two screenshots above) is a modular-arrangement open-source realtime software sound synthesizer for linux. It opens default as a monophonic synthesizer but polyphony can be obtained by opening it in a terminal and typing
ams --poly [number]. There is a wide range of modules that can be loaded in AMS, including various envelope modules, various cc control modules, and depending on the plugins installed, many types of LADSPA effects.
AMS has a very useful preset save and load function (shown in the top of the middle screenshot above), which allows to make multiple presets for each patch arrangement. Of course, if the patch is modified, it will alter the presets accordingly!
AMS also gives plenty of continous control of midi signals, which can be bound to any of the parameters available in the current patch arrangement (shown above in the screenshot to the right). By moving the desired knob/slider on your midi controller, the dedicated parameter will appear in the lefthand window; select the synth parameter in the righthand window that you want to be controlled, then click "bind" and they will be linked.
A few presets: AMS Patches 01
A nice collection of patches for AMS, made by Ken Restivo: Restivo. Very good quality. (However, not all of these patches work properly in the newest version of AMS.) Many presets are available in the demo instruments that are included in the installation package: just click on "View" and then "Parameter View." The presets can be scrolled by clicking on the -1/+1 buttons. Any newly-made preset can be saved by clicking "Add Preset" and it will be added to the first empty preset slot.
Alsa Modular Synth Website