After long reading about transformers I'm about to venture here...
I am not own the audiophile grade 449x DACs but my answers are based on my own experience with AK449x DACs and on the DIYers/clients feedbacks. So let me comment
1. My custom DAC boards use AK449x in mono mode. With active filters one does a tradeoff between noise and distortion to sum the two channels with resistors into the high input impedance filter. Did anybody check with AKM DACs in mono mode with transformers for optimal resistor values?
I wonder how big the difference in sound quality is when deviating from the recommended load impedance?..
Mathematics should be simple. If it is indicated as 10k for a single channel, then it should be OK for 5k per 2 paralleled channels. But anyway there is no the strict relation between the impedance seen by DAC and the SQ after my transformers. Trafos will be simply better regardless of your angle of view as trafos for the AK449x are 1:1 and they do nothing with the impedances seen by DAC.
2. Theoretically it may be OK to disable the DAC internal oversampling filter and run it NOS. This however is not easy to accomplish with standard front-end sources without e.g. some FPGA/DSP intervention. Is the expected performance of this AKM DAC series with enabled filters competitive with the alternatives from ESS/TI when using transformers, or is it near-mandatory to run it NOS?
I am (still!) feel dissonance when someone joint Sigma-Delta DAC and NOS word close together. NOF should be OK abbreviation probably, but NOS???
Nevertheless... I can refer to the PCM1794 DAC (dddac) which works best in so-called NOS mode (even better from HQP filters btw but it is another story).
But again, anyway there is no relation between NOS/OS/DFs and my trafos, you should rely on to your own prefferences, my trafos are are suitable with any mode.
3. Application is a 3way active project i.e. six total channels. This disallows using the JFET buffers because clean enough separate power supplies are too complex. However, passive implementations have shown that the bass usually suffers the most. Would it be an agreeable assumption/compromise to use active on the bass and passive for mid & tweeter?
As I said, trafos for 449x are 1:1, so the output impedance from your DACs will not be changed and it is low enough as I understand for most of systems (600R-1k if I'm not wrong). In this regards you simply should be sure that your LF responce is OK within your system even without trafos, and then simply use my D60 trafos, they will not downgrade your LF response. Actually the LF will be even better with my trafos due to its overall SQ superioty against the OPs.
Thanks.
Ivan.