I don't have any experience with the Westones, so I have no observations to offer pertaining to them. What I can say, though, about the Grado HF2 and the Senn HD600 is that the MAD Ear+ HD drives them both really well. And here's the nice bonus - they are both driven really well using the same tubes, in my experience and estimation. So, no need to stock up on different manufacturers or vintages of tubes to optimize each of them. Nope, the same tubes which bring out the best from the HF2 will also work really well with the HD600. While I've touched on the subject of tubes, for the past few years, my favorites in the MAD Ear+ HD are: a Sovtek 12AX7 sold by sandyphoto on eBay, usually with the title - "Best 12AX7 I've ever heard - No Really!" And the 12B4A tubes that I've liked the best so far are: NOS RCA or Marconi. I purchased one set of what "National Electronics 12B4A's" because they had a red label on each tube. I was hoping to purchase a pair of GE Red Label 12B4A's (which marroyo has recommended a few times), and I was hoping that these Nat Elec Red Label tubes might be (because of the red label) rebranded GE's and offer me the same goodness of which marroyo had posted. And, to my pleasant surprise, I heard an appreciable - but more subtle than striking - improvement in sound quality using the Nat Elec red label 12B4A's. Unfortunately, I've looked a number of times since, and I can't find more of the Nat Elec red label 12B4A's or any of the GE Red Label 12B4A's. So, when this pair I've got goes, I'll replace them from the stock of NOS RCA's and Marconi's I've laid in and keep on listenin'.
But, I'll end on the topic at hand - to my listening, the MAD Ear+ HD works really well with both Grado HF2 and Senn HD600 phones - with the same tubes used in it for both phones. What I've just posted for the HD600 does not also apply to the HD650 (as I've also previously posted). The MAD Ear+ HD is not my favorite amp for the HD650 - the Purity Audio KICAS Caliente is - so that's not at all an issue for me.