I heard the O2 mk2 (used to be mine...) with the BHSE at the Meet. I thought it sounded simply excellent. I also think that the Mk2, with its slightly more pronounced midbass (to my ears) is a better Meet phone than the Mk1 because it can overcome some of the ambient noise a bit better. Regardless, I thought that the BHSE took the Mk2 and gave it the kick in the pants that it needs.
Bear with me here, I feel like that the Mk2 is just slightly, um, lazier than the Mk1. Just the very littlest bit slower than the Mk1--perhaps due to te midbass. And in some ways, when I was comparing the two, I found this to sound almost like the Senn veil, but to a far lesser extent. We are talking about very small differences here. So, when I had the Mk2, I found myself wondering if there was an amp that would be able to "punch through" that bit of veil. I feel like the BHSE is such an amp. The Mk2 sounded nimble and all around great.
Unfortunately, by the time I got around to trying the mk1 with the BHSE, Nate had to break down his equipment to give it some time to cool down (for some reason Nate didn't want to risk his packing boxes bursting into flame). So, I don't have those impressions. But as Nate and I discussed, the differences between the phones may be obvious to Stax lovers, but they are much much closer than they are different.