the default filter (by-pass) is dynamically constrained with less than stellar bass. having re-read alan taffel's review of k-03, i switched to apodising 1 (S_DLY1) which was night&day better.
but i did not try all the filter and up-sampling options because there are altogether 20 combinations (5 filters times 4 up-sampling options) and each time, imo, one needs to play with the toe-in and speaker position to get the best sound. so it is a colossal work.
while the default options definitely are not the best, figuring out the right combo is something i found very difficult to do. so views and comments are appreciated.
my current setting are S_DLY1 with 2x and i continue to work on toe-in to tame too-pristine HF.