Linux users, this looks promising!

MusE is a multitrack virtual studio for Linux that has support for sequencing of both midi and audio and has, among other things, support for LADSPA, Jack and ALSA.

I saw this and was pretty impressed, though since I'm not on that platform I can't test it out. :( But in addition to the many good features, I note that you can run PC VST soft instruments, meaning that there are a good number *more* microtunable instruments one could use with Linux.

http://lmuse.sourceforge.net/

From the site:

Some Highlights

* NEW ReWire Support
* NEW ASIO 2.0 Driver support (via ALSA)
* NEW Automation architecture for both midi and audio
* Improved Better midi controller abstraction: available midi controller can be defined in instrument definition file (*.idf)
* Improved Integrated mixer with:
* NEWUnlimited number of inputs
* NEWUnlimited number of outputs
* NEWUnlimited number of tracks
* NEWUnlimited number of aux's
* LADSPA effects on all track types, both before and after fader
* Stereo/Mono track types
* Improved interface changes, the TrackInfo pane is used more
* Improved Midi instrument definition files (*.idf)
* Improved Lots of customization options
* NEW Shortcut editor
* NEW Drag and drop import of midi files
* NEW Drag and drop import of wave files
* Standard midifile (smf) import-/export
* RTC (Real time clock) use for rock solid timing
* Advanced arranger with track and part abstractions.
* midi editors:
* Pianoroll,
* Drum editor,
* List editors,
* Controller editors
* Realtime editing of "everything"
* Unlimited number of open editors
* Unlimited undo/redo
* Realtime and step-recording
* Multiple midi devices
* Unlimited number of tracks, both midi and audio
* Audio playback/recording
* Sync to external and internal devices/applications:
* MTC/MMC
* Midi Clock
* Master/Slave
* NEW Jack transport enabled
* LADSPA effects
* Integrated softsynth architecture M.E.S.S
* Several internal synths available
* Simple architecture for adding more synths
* NEW support for industry standard Windows VSTi softsynths
* JACK enabled
* ALSA sequencer client
* LASH enabled (formerly LADCCA)
* Raw midi device usage through ALSA
* XML based project files
* XML based configuration files
* project file contains complete app state (session data)
* Application spanning Cut/Paste Drag/Drop
* uses C++, QT3.2 GUI Library, STL

And the most important feature for a free application:
* GPL Licenced
Cheers,
Jon