I have it always paired with the the arrow 4t. I've tried it once with the Fidelio x2 and the difference for me was noticeable, more than expected, the sound felt significantly opened. With my Flare R2a, a harder than expected iem for weak sources, the arrow improves also the sound, drives them better. However with my sennheiser ie800 difference is not dramatic, the Plenue could be enough for them even though they find more clarity on the arrow. Finally with my Audeze Sine is essential, there's not enough juice to move them in the Plenue.
So, from my experience, the Plenue is not weak but only fullfills easy to drive iem's demands. Bigger cans, or any kind of headphone that scales up well with better sources will react inmediately to the presence of the headstage arrow, which offers a very good pairing with the plenue, the best i´ve tried. With it paired i don't touch the plenue's eq, by the way, just the bass and treble switcher as i prefer hardware eq and the sum of both has a double warm effect over the sound which is too much for my taste.
Hope it helps!!!
(Edit: didn't notice the date of your post...hope it helps anyway!)