"F4" ("= slow roll off, minimum phase Not linear phase in the pass band. Best for acoustic music without compression and artificially high levels of treble. Will have very low level of ringing caused by transients and ringing will be shifted to after the transient. )
So the slow roll off lets enough imaging occur to replace the hashiness of compression on music that doesn't have it yet. And naturally the post transient filtering (which imaging tones along for the ride) are so much nicer. I believe it is J-J that always calls this a broken design or a broken DAC. Wished they (and all the others) would just leave this off.