david,
Number 1 on your list messes this up, but closest is the HA-2. I am of the opinion that the Sennheiser HD600 isn't good enough to show the amp's greatness. The Meier will, however, make the HD600 sound good.
Crossfeed on the HA-2 is interesting. When listening to "stereo" mixed originally in mono, there's more fill and guitars sound somewhat realer. I like the kinda on option most. The crossfeed on The Headroom Max w/ stepped attenuator sounded like a tube effects switch - warmer, fuzzier, doesn't sound as good as tubes. I personally think the Max is too good for it, and I would have hated it with the HA-2. I think the Meier crossfeed is pretty good.
HA-2 excels is the midbass and tonality, never too much or too little. Little cold at times. Since it is fast, clean, and punchy in the midbass, the deep bass just doesn't need to slow it down. HD600 accentuates the bottom and has a thick middle: I think you'll get the deep bass you want and it will be balanced with the rest of the spectrum.
Depending on what "decently large headstage/soundstage," know the Meier can't throw sounds real far; instead, it just layers instruments relative to one other in a close area. If big soundstage is your top priority, you need to make a new list - I'd guess the RKV would be on top, based on its reputation.