CD has laid back and uninvolving sound

Chris, I never tried long plate Mullard's in the AA DAC, regular Mullards were way to "dark/romantic" lacking extension and dynamics for me, but Amperex long plate D-getters give me musical pleasure to a high degree. Recently, in my new preamp, I have put in 12AU7's Mullard long plate

Chris, you and I have the same digital front end. Yet, I find this combo to be wonderfully musical with great microdynamics and full extension on the top and bottom. Very "emotionally" involving, not laid back and distant at all.

I do use the MBL reference clamp instead of the one that comes with the 1621 transport, the tubes in the AA DAC are Amperex long plate D-getters, and finally I use a Stealth Sextet cable. Would you please share if you have ever experiemented with the upgraded clamp and what tubes and cable you use in your system. Maybe, its these other factors that are taking away the dynamics/emotional aspects that you say you are missing in your rig. I have heard most of the very high end digital gear and still have not heard anything that makes me want to replace either the MBL transport or Accoustic Arts Tube Hybrid DAC in my system.

No offense to you Chris, but after spending alot of money for a world class reference DAC, to leave in crappy tubes made in China is pretty stupid/silly to me. What's the "hassle" about six screws and putting in two tubes?

I also believe you are way understating the importance of how the AA DAC performs depending on what tubes are being used. So, if you think tweaking means changing stock crap tubes with musical NOS bottles, then do yourself a favor and stick to SS pieces. I also recommend you don't try different power cords, footers, isolation devices because these are tweaks that also would be to much of a hassle!

You also did not comment on what clamp you use or what digital cable, which could be an important set of factors. On the other hand, useing the stock tubes probably makes these other factors mute anyways.