Nicdob hi.
Can't believe nobody has asked this yet.
But have you tried the 4ohm speaker tap on the amp?? You will loose half the wattage, but what's there may drive the bass much better. Triode mode/4ohm tap is it's lowest output impedance, which may fix the bass problem, but leave you short on big loudness levels.
http://www.stereophile.com/content/cayin-50t-integrated-amplifier-measurements#fMrbLoy4HGiKxcxV.97
As if these speakers dip down to 4ohm in the bass and have a heavy negative phase angle as well this will kill the bass of the amp.
Cheers George