I don't hit play in QjackCtl at all. I always start the QJackCtl app, then other bits and pieces like Yoshimi and Qsynth, then hydrogen, then Rosegarden. In Rosegarden, make sure you have been to Edit / Preferences and in the "Behavior" tab, check the "Use Jack Transport" option. Hitting "play" in Rosegarden drives the drums in hydrogen. Well, that's my workflow anyway.
I've used arpage a couple of times in the past with the method you describe with Qtractor - I ended up using Jack Capture to record some stuff from it, then took the resulting Wav file into RoseGarden. But I ended up moving away from arpage (distracted more than anything) when I discovered Qmidiarp. I'm not saying Qmidiarp is better than Arpage, just different. I guess that's one of the issues with Linux audio - sometimes there are too many pieces of software! But that's good though!
If you're interested in Qmidiarp, I think you have to install via synaptic - can't remember - I don't think it's part of the 5.0.3 iso. I use it in the same manner to arpage i.e. create something at the tempo I want and record it to a wav using Jack Capture. I use these steps (probably a better way, but works for me) Start QJackCtl, then A2JMIDIDI, then QMidiarp (type "qmidiarp" from a terminal screen or "qmidiarp -a" to use ALSA) , Yoshimi, and the Virtual Keyboard (yep, I don't own a real keyboard). Then in Patchage, I create spaghetti junction -
(hope I get this right - this is using qmidiarp via ALSA i.e. qmidiarp -a )
-> connect the red qmidiarp outs, to the Yoshimi in
-> the blue left and rights in yoshimi, to blue playback
-> for virtual keyboard, the red VMPK output to red Yoshimi in
-> the green VMPK output to green qmidiarp in
-> then flick to the Qmidiarp app. Click the left-most green plus symbol to create a new Arpeggiator. Fiddle with the settings below the green screen, choose your tempo at the top then hit the blue play button.
-> flick back to virtual keyboard, press a key and you should get something. The other green buttons at the top let you create your own sequence - such fun! Can be a bit flakey - tempermental if you change settings on the fly.
Of course, the other option you have for arpeggios is to use the step-write capabilities in Rosegarden.
I will be interested to read comments from others, more experienced than I, re Arpage and qmidiarp. There's always something new to learn.