You could well be right. Can't say I've seen all that much of Mkhitaryan but when I have he's always disappointed me. As for defensive duties, you'll find that Firmino trumps Mkhitaryan in tackles per game. That's one area of Firmino's game that I've always been impressed with, he tackles very well. I rate his pressing game too, he certainly is not lazy. Bit too easy to say he's defensively crap being part of Hoffenheim tbh.
Oh, he isn´t, but he is clearly weaker in that regard than Mkhitaryan, at least for the purposes of Klopp. Averaging more tackles per game means very little, because first it ignores the approach of the opposing team, which is vastly different and second the focus of Dortmund´s pressing system. Our system is not focused on tackling. This can be easily seen by looking at Kevin Großkreutz, who averages just as much tackles per game as Firmino this season despite playing the majority of it as RB. This tells pretty much the whole story.
Our pressing system is based on movement, both in sheer quantity and purpose. Its about closing down oppenents, swarming them, limiting space to pass and run and ultimately forcing errors. The primary pressing player does very often not get a benefit of it in terms of stats, but the one who capitalizes (in the case of the CAM the CM/DM most of the time) on the errors.
Also, you name Volland yet he's actually pretty poor at his defensive duties, got a much lower tackles per game, fouls more. Barely intercepts the ball.
Think you have it the wrong way round! 
No. I just take their respective roles into account. Firmino played the whole season for Hoffenheim pretty much exclusively as CAM, while Volland is a wide player, who can also play as CF.
As a CAM in Klopp´s system the defensive duties are focused on positioning and anticipation, both areas in which Mkhitaryan is the best on his position in the era Klopp, even better than Kagawa, who was already stellar in that regard.
The wide players´defensive duties are easily named: backtracking. A lot. Looking at that, Volland ran pretty much as much as Firmino, who I simply don´t see on the flanks.
The biggest reason why I could see Volland fitting into Dortmund more than Firmino is his body language. He just seems to be totally commited all the time. This is something that reminds me a lot of Reus in his Gladbach days. It is not about what Volland brings on the table right now defensively, it is about in what player Klopp could turn him into if he gets his hands on him. There I see simply more potential. We are scouting him for several years now because of it.
