OK, had a look at the schematics...
You can actually maintain the opamps there. I would disconnect the discreet front end and wire the opamps as a unity gain balanced line receiver, and maybe you can maintain the attenuators so that you can feed it with higher level signals.
You could also try exchanging the TL074 for some BB chips, like the OPA404 or any other nice FET input one.
Also, I would sub the 22uF caps for some really good ones. Maybe you could experiment with some smaler films, like 4u7/50v, if you can find it.
Wait!
As the converters are balanced input, I don´t see why you couldn´t try bypassing all the active preamp electronics all the way...I mean, why would you need a balanced line receiver??? I don´t see any bias at the inputs, so you could also bypass the 22uF caps all the way...(not sure, wait for confirmation, or read the converters datasheet gefore trying something like this).
You could try to conect the peak meter buffer to the AD inputs directly, so that you still have some cliping lighs...