This is a very long way from being evidence, on several counts, but as regards early references I found this in 'Conversations with Wellington.' It is from 1839, but the Duke is repeating what he heard during the campaign. Wellington got on alright with Soult after the war- he had no axe to grind.

"Soult's officers were great plunderers. The spy I have mentioned told me that in the French army he was once sitting at table with Soult after supper, when General Eblé remarked ; Le fait est, Monsieur le Maréchal, qu'il y a seulement deux honnetes hommes dans l'armée; le premier c'est Latour Maubourg (who sat opposite) et le second c'est moi. Soult sat quiet and seemed to acquiesce in his own exclusion."

'I remarked that the degree of French plunder was clearly indicated by the enormous amount of booty and baggage taken at Vittoria. "It was enormous, certainly" said the Duke. "We got about 1000,000l from the military chest, .......Not all Spanish;some had very lately been sent from Paris to pay the troops, and we took it .... The baggage and encumbrances of the French army were immense at Vittoria. A great many ladies too! One of their prisoners said to me after the battle, Le fait est, Monseigneur, que vous avez une armée, mais nous sommes un bordel ambulant.

I know that last bit isn't relevant to the discussion, it was just too good to leave out!

regards,

Susan