Ah, this is silly. Don't you know all amps sound the same?(Seriously, though, thank you for taking the time to give it a good listen--and for the feedback!)
I hope you understand even if you don't agree. Errr, I mean: I hope you agree even if you don't understand.
My quest to source the M51 locally didn't bear fruit so I now have a Gungnir preordered and paid for. If it's merely a slightly refined Bifrost it'll be a very good match for the Mjolnir and I'll jump off the headfi merry-go-round with contentment and great relief.
YW.One thing I should add is that thinner sounding DACs like the Benchmark, or treble tilted headphones may not be a good match for the Mjolnir. I'm using the PWD2 fw 2.02 which is fairly warm sounding. YMMV. Musical preferences and recording quality also play a huge role. The Mjolnir really started to pull away with well recorded classical.