With two AHB2 (in mono configuration), the sound quality has changed considerably. Tracks which sounded thin earlier, were fuller with more ‘body’ and warmth.That to be expected with half the damping factor and double the output impedance, that will always make the bass and lower mids sound richer.
Now try vertical passive Bi-Amping with the both amps back in stereo mode, and prepare to be amazed!!!
http://www.av2day.com/wp-content/uploads/2014/05/biamp2.jpg
Cheers George