I have had the Dartzeel CTH8550, Boulder 865 and the Vitus SIA-025. In fact have both the Boulder and Vitus at the moment.
I found the Dartzeel to have the sweetest sound but lacked the ultimate dynamics I expected and was used to with my CJ pre and power combo. The Boulder has dynamics in spades. Its the most neutral of all the three amps and has plenty of attack and drive. Lovely sound with warmish sounding speakers I have found. The Vitus, as Melbguy1 states, would be my pick for a sound that is liquad in the midrange, very dynamic and has bass to die for. It is noticeably warmer than the other two amps.
Synergy is the key though which I cannot comment on really, having never heard Hansen speakers.