Thanks all, I just checked a bit re the recommendations. Adding a subwoofer is not an option at this moment. Specifically, a pair that can go down towards sub 50 Hz would be ideal. The 6-inch ones are just too big (typically >30 cm depth).
BTW: my source is PC --> t+a dac, indeed I am using DSP rolling off from 1k - 20k ( -3db -- - 5db). 607s do not sound fatigue that much, but I have no clue at what cost. .....

