Now lets say my favourite snare sound is on my old Korg 03R, but I love the HHat sound from Toontrack Vintage - on a decent DAW, the ability to use both should be a relatively simple task.... nooorrt.
The very simplest way would be to set up two tracks, one "playing" the Korg, one the Toontrack, and put the notes you want each to play on that track. If you want to work with patterns in a Drum Edit page, you'll need to define a Drum Map (or whatever your particular DAW calls it.)
There aren't really MIDI-only sequencers any more. But Cubase has all the MIDI it ever did, plus more. I'm sure its competitors do too. The "alternative" approaches of Ableton, Fruity Loops etc. tend, I'm afraid, to come down to merely restricting choice of functions rather than offering revolutionary new ones.
Reaper does what it does. Like Linux applications, it gets over-praised.