I don't think of any provision in the Constitution in terms of being a "federal power grab," but these Civil War amendments did provide the Congress with some very substantial new powers.
Consider, for example, the Fifteenth Amendment. Section 1 of that amendment prohibits any state from discriminating against people "on account of race, color, or previous condition of servitude." Section 2 empowers Congress to enforce the amendment by appropriate legislation. In Lassiter v. Northampton County Board of Elections (1959), the Supreme Court upheld state literacy tests when it was contended that they were unconstitutional pursuant to Section 1 of the Fifteenth Amendment. Thereafter, Congress determined in the course of legislative hearings that some states were utilizing literacy tests for the purpose of depriving blacks of the right to vote. Based upon its findings, Congress enacted the Voting Rights Act of 1965 which barred certain states from using literacy tests as a qualification for voting. The Supreme Court, in Katzenbach v. South Carolina (1966) held that, pursuant to Section 5, Congress was empowered to ban the use of literacy tests in states where Congress believed the tests were being used to evade the requirements of the Fifteenth Amendment.
Any way you look at it, that's Congressional power.