The Soult Collection is a subject of a long, long debate over the years....
This has been recently (2000) rather extensively discussed by his biograph Nicole Gotteri and I would strongly recommend to read it before drawing any conclusion about this subject.
An example : six of the famous spanish paintings of his Collection were actually given to him by Joseph, when Soult came to Madrid, at the end of 1809 (one Velasquez, one Ribera, one del Piombo, one Titien, one Van Dyck)
Gotteri remarks also that many of the paintings of Soult Collection were in rather bad state of conservation.
Another french author, François Malye, in his recent (2007) book on the Spain War, "Napoléon et la Folie espagnole", writes that "Soult essaya toujours de démontrer que ses acquisitions avaient le plus souvent dans des conditions légales"
As I said : a long debate !
Best Regards
Robert