I read about this review on another forum, that's why I signed up and that's why this is my first post here. I own a TDAI-3400 so everything that is written about it has my special interest..
@amirm my first reaction after reading your review was, could it be that you had a faulty unit on hand? Especially because some (unexpected) outcomes only concerned one channel. Did you confront/consult Lyngdorf about your findings?
Secondly, I think it is a strange decision to review this unit as a 'traditional' DAC. Almost no-one will use a TDAI-3400 as a DAC to drive an (analogue input) amplifier. I think that - given the concept - it is not strange that the output stage / D-to-A is not of the highest quality. In 9 of the 10 instances these outputs will only be used to drive subwoofers via its cross-over function. The TDAI-3400 is essentially a powerDAC and excels in driving speakers via its speaker outputs. I actually compared the TDAI-3400 to NCore, Ayre, Devialet amongst others and preferred it. By ear but nonetheless.
That said, I use (and love it) as an amplifier, very good DRC and as a cross-over but I drive it counterintuitive with a Chord DAVE via its very high quality analogue inputs (optional module). This easily bests feeding the TDAI a digital signal. Best digital inputs by the way are AES/EBU and S/PDIF. USB and ethernet are not best-in-class so not strange that your test via USB did not measure well.
Lastly, I wonder if it could be that the Lyngdorf ICC (Inter-sample Clipping Correction) function did blur your outcomes and measurements. In my (very resolving) setup ICC 'enabled' degrades the SQ. I notice a relay switching when toggling this function so I suspect that ICC puts extra electronics in the pathway. Also the subsonic filter that Lyngdorf implemented could be a factor. Both ICC and the subsonic filter are enabled after a factory reset, as you did.