Deep bass response of Audio Note 2.1x Balanced DAC

I've had this DAC for the past month and have been very impressed with it overall except for its seeming (in)ability to reproduce very deep reverberent bass, mostly from an acoustic bass. I hear--oddly not all the time--distortion and smearing from standup bass, in mostly jazz combos. I've changed the stock tubes,and reverse phasing it, but I'm finding it more and more musically unsatisfying when the sonic picture is marred by tubby or smearing of deep bass notes. Since my preamp allows me to switch inputs to hear the output of the CD player near-simultaneously, it is clear the roll-off is pretty pronounced from the AN. A significant flaw (?) in an otherwise fine DAC. I hear it when the transport is either my Sony XA777ES or my cheapo Pioneer DVD player, so don't think it's transport related. By the way, the dealer says this is the trade-off for a tube-based DAC. Is this common with others? Curious as to others experience with AN DACs, especially with deep bass output.