A Keynesian macrotheory shows how the level of income varies with the resulting investment. Despite a famous aside by Piero Sraffa, I do not feel comfortable in ascribing this theory to him. The rate of profit is determined by the interactions of investment, profit, and savings. Given fixed savings rates, the distribution of income is determined by the rate of growth, which, in turn, is determined by investment. Two equations determine the two unknown quantities the rate of growth and the rate of profit.
Prices fall out in the wash.
Investment is not constrained by savings, but savings adjusts to the investment decisions of capitalists. Consumption per head is known given the technique and the rate of growth. This is a debated point. Whatever it may be, it cannot be equated with marginalism. Marginal techniques are the common possession of all theoretical economists.