Simple LInux MIDI sequencers may be adequate for many users. But it takes a while to get used to any sequencer interface, and when users find that a sequencer can’t do everything they need to do, they’re forced to learn a whole new interface.
In MMML#1 and #2 I checked out the QTractor sequencer. It’ll do a lot of useful stuff. What it doesn’t do, though, is let users access ALL CC (continuous-controller) messages for editing. For those who want to be able to record pitchbends, filter moves, LFO moves, patch-changes, pedals, or the output of MIDI-controller hardware … and save and edit ALL of that input … a more versatile sequencer is needed.
The MusE sequencer has been around for a while (more than 10 years). It’s still being maintained and improved. “It was originally written by Werner Schweer and now is developed by the Muse development team.” I’ve seen it referred to online as ‘the best MIDI editor available for Linux’. One thing it certainly does is give complete access to CC-editing.
The manual page called Window Reference Guide shows lots of images along with descriptions of most sequencer operations. The section called “The Pianoroll Editor” gives a glimpse into how CC’s (velocity in this case) can be edited.
One other thing: while MusE handles audio tracks as well, that capability may not be needed. It can be started without loading audio support by opening a terminal window and typing $ muse -a.