Does anyone have a source reference for Advayavajra in tantric buddhism? This article on tantra -- Buddhist Tantra - 5 -- says 


  The deities of the vajrayAna are all manifestations of shUnya.
  Advayavajra says in a very characteristic verse that the deities are
  nothing but manifestations of shUnya and are by nature non-existent,
  and whenever there is manifestation it must be shUnya in essence. The
  process of evolution of deities from shUnya has four stages: the first
  is the right perception of the shUnyatA or voidness, the second is its
  connection with the germ syllable, the third is the conception of an
  icon and the fourth is the external representation of the deity. This
  statement which occurs both in the sAdhanamAlA and in advayavajra is a
  very strong argument against the theory that later Buddhism was
  nothing but gross idolatry.
  
  They are rather the voluntary manifestations of the shUnya in
  accordance with the bIjamantras uttered by the worshippers, with an
  appearance suitable for the function he has to discharge.


Does anyone have the appropiate references to these statements?
If by Advayavajra they mean Maitripa then it must come from Maitripa's Amanasikara.

Specifically, in Mahasukhaprakasa, text #16 in above translation, so pages 181-187.

In addition, for an English translation of a small but relevant part of Sadhanamala see "Two Tantric Meditations: Visualizing the Deity" by Luis O. Gómez. in Buddhism in Practice pages 236-245.