I'm struggling to find any reference to using an allpass filter in a circuit that uses only differential amplifiers?
Is it doable - anty pointers?
Can't say that I've ever tried this with an allpass filter, but this article describes how to transform any single-ended MFB design into a fully-differential one. Might be worth pursuing - take the design from FilterPro and use the procedure described in the article to make it fully differential.