The reason there's few succinct sources is because the effects are all over: stagnant wages, outsourcing and offshoring of high-paying jobs, boosts to stock prices after layoffs, etc.
http://www.epi.org/publications/entry/bp195/The thing to ask them is
why they think tax breaks for high-income people will stimulate the economy?
They're obviously old enough to remember Reagan. Do they remember Reagan's claim that by lowering the tax rates, people would use that money to modernize out industrial plant? Part of the conventional wisdom back then was that every other major manufacturing nation had suffered massive destruction during WW2, so most of their factories were newer and more efficient than ours. Modernizing them would help turn around the problems that were turning the Steel Belt into the Rust Belt.
The wealthy got the tax breaks, but what happened? The money went to Wall Street, and what we got was Mergermania. That stimulated stock prices, and made a lot of money for a few people, but ordinary people were just "fat" that needed to be "trimmed". The modernized factories were mostly built in Mexico and other cheap-labor countries.
The same bait-and-switch sales tactics happened again and again: LBO's, the New Economy, etc. The claim is that this is needed to get ahead of the ball, but the benefits never quite reach the lower levels.
Here's the point: why should they? Making money by real investment (developing a new product or process, or providing the resources to those who do, and getting a piece of the action on the money made from that) involves hard work and real risk of it not panning out. What's a much easier and surer way to make money? Squeezing people: cut wages, cut jobs, make people work longer hours without added pay, sending jobs to cheap-labor countries.
All Wall Street cares about is the numbers on the quarterly report, so it doesn't care about how those numbers are gotten. Real investment in development takes a back seat to speculation and liquidation. You can get real fat from eating seed corn, but only if you don't mind a lot of people starving later.
Go to
http://www.barlettandsteele.com/ and look up some of the stories and books these journalists have written over the last 20 years. Try corresponding with them and with the people who've written some of the articles you've found that were too short to include the actual data. They may be able to help point to the data AND to things that better describe things in layman's terms.