"However, I'm not sure I see any actual evidence to support the power theory."

I am a little unclear on what you specifically mean by "the power theory".

Do you mean to question the proposed relationship between greater monetary resources and greater (political and social) power? This is something that seems pretty obvious but there are some interesting graphs presented in:

"Economic Inequality and Political Power (Part 1 of 3)", Martin Gilens, 2012. http://themonkeycage.org/2012/08/economic-inequality-and-pol...

"When preferences across income groups do diverge, however, I found that the association with policy outcomes persisted for the affluent but disappeared for the middle class and the poor, as the second chart shows...If federal policy more equally reflected the preferences of all Americans, we would see a more progressive tax structure, higher unemployment benefits, stronger regulation of business and industry, a more protectionist trade regime, more prayer in public life, and less access to abortion."[1]

Another source is:

"Democracy and the Policy Preferences of Wealthy Americans", Benjamin I. Page, Larry M. Bartels, and Jason Seawright, ?. http://faculty.wcas.northwestern.edu/~jnd260/cab/CAB2012%20-...

"There is good reason to believe that the wealthiest Americans exert more political influence than their less fortunate fellow citizens do. Historically oriented scholars like
Thomas Ferguson, William Domhoff, Fred Block, and
others have long argued that “major investors” or business
elites dominate the making of public policy and the agendas of both the Republican and the Democratic parties.1
Jeffrey Winters maintains that the top one-tenth of 1 percent of US wealth-holders constitute an “oligarchy” with
decisive power over certain key policy areas related to what
he calls “income defense.”2
Jacob Hacker and Paul Pierson describe how Washington has “made the rich richer.”3

"Even without being able to gauge the actual political
power of wealthy citizens, we can confidently reject the
view that extensive political power by the wealthy would
be of little practical importance anyway because their policy preferences are much the same as everyone else’s. On
many important issues the preferences of the wealthy appear
to differ markedly from those of the general public. Thus,
if policy makers do weigh citizens’ policy preferences differentially based on their income or wealth, the result will
not only significantly violate democratic ideals of political
equality, but will also affect the substantive contours of
American public policy."

Which I found from, "Wealth Inequality and Political Inequality" by Bruce Bartlett. http://economix.blogs.nytimes.com/2013/04/30/wealth-inequali...

Or, do you mean to question the proposed relationship between monetary inequality[2] and (political and social) power specifically? There are several links on the Wikipedia page concerning specifically inequality and social cohesion, crime, and social, cultural, and civic participation; all of these are more or less linked to perceived social and political power. However, more specifically:

"Economic Inequality and Democratic Political Engagement", Frederick Solt, 2005. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.602...

"It finds that income inequality has a large
negative effect on political interest, frequency of political discussion, and participation in elections among those with median and lower incomes."

Another source is:

"Income Inequality and Participation: A Comparison of 24 European Countries", Bram Lancee and Herman van de w
erfhorst, 2011. http://www.uva-aias.net/uploaded_files/publications/DP6-Lanc...

"To conclude, although resources are important to explain participation, there seems to be support for the theory
that resources are not fully able to explain inequality effects on participation. In addition to the material or neo-material theory that explains inequality effects in terms of resources, we found support for the idea that inequality
affects people of different income levels differently through non-material, inter-individual processes. "

Also, Martin Gilens' subsequent blog posts provide further discussion of the effects of inequality specifically.

[1] A blog post, yes, but I believe the data is taken from his book, Affluence & Influence: Economic Inequality and Political Power in America.

[2] Yeah, I'm assuming income inequality is strongly related to wealth inequality, at the very least.