Digital Audio Workstation: alternatives to Logic Pro or Cubase for Ubuntu?
Ardour is what you are looking for.
You can install it easily from Ubuntu Software Center (shortcut link: ardour ), or by running
sudo apt-get install ardour.
Does it suit your needs? I guess so.
Routing MIDI signal - yes, this is definitely possible, you can also easily route audio between applications and hardware sources/outlets. If you are looking for advanced MIDI features, like recording and editing, you may want to try out Ardour3, which is in Beta phase, and available to try at Ardour website.
Recording Line In audio - Works perfectly! You can record unlimited number of channels, route them through effects and process them however you wish.
Editing, combining, processing tracks - That's what Ardour was designed to, and it does it has wide possibilities in this matter. It provides both simple tricks like mixing, moving, transposing tracks, as well as advanced signal processing tools. You should see yourself if there is everything you need, but I believe it is so.
I think it is also worth to mention that Ardour is probably the most acclaimed and recognised DAW for Linux, which is why you should at least try it.
Of course it has many other features you may enjoy, either find a list of them at the website, or install it and give it a go :-)
Also, Its better to try LMMS (Linux MultiMedia Studio) which can be able to compose virtual music, an equivalent to FL Studio. Its completely free and Open Source DAW (Digital Audio Workstation). Try installing Plugins. It comes close to Professional Music.
Else, Probably the best way to compose Professional Music Sounds is by using a score software like MuseScore and using a soundfont like sgm or generaluser provides you the expected professional sound level. I've tried both of this.