Here is a post from Vincedog3 on the Emotiva forum form back in December. He has recently been hired on the Emotiva staff.

Quote;

"...............« Result #3 on Dec 23, 2009, 5:33pm »
P.s. the Architecture of the XPA-2 and XPA-1 are different. At the input for instance the XPA-1 is quad differential and also it is fully balanced. As from the website
FEATURES:

• True Differential Reference™ design.

• Fully balanced quad differential input stage.

The XPA-2 is Dual Differential, and the balanced input is summed. You get the benefit of common mode noise cancellation, but the XPA-1 is fully balanced.

I heard both the XPA-1 and the XPA-2 both are great amps, but the ease and clarity of the XPA-1s make me want them................"

I also read that that the UPA-1 was not fully balanced.