Bottlehead Crack with speedball upgrade. Haven't heard the WA3 (which is comparable to a DV336) but have heard the other two mentioned amps. For the HD650 it's a no brainer you won't get a better synergistic match with the Crack and any of the high impedence Sennheiser can's namely 580/600/650 and 800. The Crack + the 580-650 headphones is the perfect couple. /thread.
I was curious about the crack myself. Would you say it's worth upgrading to a Crack without the Speedball upgrade from a Burson HA-160, or would the improvement be marginal?