The multi channel amps I would recommend would be the Sim Titan and the Plinius Oden. However, with that in mind, I think the Pass amp you own would be a better choice for stellar two channel versus the Multi channel amps. Sim is coming out with a multi channel 18K amp.
Here is the link.
http://www.simaudio.com/xmoonmc8.htm
Good Luck!
Here is the link.
http://www.simaudio.com/xmoonmc8.htm
Good Luck!