Krishna as avatarin?

[Martin Gansten]

This query is being cross-posted to Indology and RISA; my apologies to
those who receive it twice:

While the Puranas and Mahabharata portray Krishna as an avatara of Vishnu,
Gaudiya Vaishnavas take the opposite view and regard Krishna as the
'original' deity, the source of Vishnu and all his avataras. I have heard
it stated that the followers of Nimbarka and Vallabha share this
theological stance, but have been unable to trace any authority for this.
Could anyone point me to one or more primary Nimbarka/Vallabha texts which
explicitly state the doctrine of Krishna as avatarin?

Thanks in advance,
Martin Gansten