Origin of the phrase "filthy rich"?

What is the origin of the phrase "filthy rich"?

As the link to Phrase Finder above explains, this phrase evolved from the phrase filthy lucre from the 1500s. Here's what Etymonline has on lucre:

lucre (n.)

late 14c., from L. lucrum "gain, advantage, profit; wealth, riches," from PIE root **lau*- "gain, profit" (cf. Gk. apo-lanein "to enjoy," Goth. launs, Ger. lohn "wages, reward," and possibly Skt. lotam, lotram "booty"). Filthy lucre (Tit. i:11) is Tyndale's rendering of Gk. aischron kerdos.

As Phrase Finder explains:

Following on the the term "filthy lucre", money became known by the slang term "the filthy", and it isn't a great leap from there to the rich being called the "filthy rich". This was first used as a noun phrase meaning "rich people; who have become so by dishonourable means".

Phrase Finder claims a 1920s origin, but the two words can be found together in print from the early 1800s. This, however, is the earliest example I can find of it being used as a set phrase rather than as a description of rich people who are physically dirty. Here's a clip from A Certain Rich Man by William Allen White, 1909 (date check):

And a final point worth noting from Phrase Finder:

As time went on the negative associations have softened somewhat. It has become to mean "extremely rich" rather than "dishonourably rich", although there may still be a trace of an unfavourable implication associated with it.