Saving multitracks record with a 11kHz LPF
I have recently received some multitracks to mix. Eight people (2 per mic, 2 per voice SATB) and one stereo pair of them all together (XY). Its supposed to be some church hymns accompanied by piano and organ. For some reason, hardware or software related (unknown) all of the vocal tracks have been recorded with 11kHz LPF. To make matters worse, from 10 recorded songs, 5 of them have been recorded with the LPF while the other 5 are "normal" and have the whole frequency range. I have tried mixing them both and the difference is quite audible.
I have tried saturation (both on inserts and parallel), various exciters, iZotope Spectral Repair with little to no success. The most success I had was Ozone 12 Exciter making it sound the most natural however when AB testing the mixes, there is still quite a difference.
Any advice is appreciated!