I usually rely on the Angiosperm Phylogeny Website (based on APG IV) for this sort of question as it is usually kept
up to date and contains modern references.
http://www.mobot.org/MOBOT/research/APweb/Here Paulownia is apparently placed in its own family: Paulowniaceae. the following is in the description:
"Previous Relationships. Paulownia is superficially like Catalpa (Bignoniaceae) and the two used to shuttle back and forth between "Scrophulariaceae" and Bignoniaceae. Paulownia has endosperm and lacks the distinctive ovary and seed anatomy of Bignoniaceae (Armstrong 1985; Manning 2000; Lersten et al. 2002); on the other hand, Catalpa is definitely to be included in Bignoniaceae. "