Bal apd is the best acrylic primer as it is much better than pva, the pva is realy just a sealer glue, where as the acrylic one puts a barrier between the wall and the adhesive, this is essensial between cement products and plastered walls as the two of them react together.
Alot of tub adhesives have now got acrylic primers built in them (tells you on the tub) so no priming is needed in most cases, every job is different and has to be addressed seperatly unfortunatly