What is the difference between the reactivity of Lead subacetate and Lead II acetate undergoing a hydrothermal process?
Compared to lead(II) acetate, lead subacetate contains additionally $\ce{2 Pb(OH)2}$. The hydroxide can either be neutralized by protons from EDTA or take part in the hydrolysis of thiourea:

$$\ce{SC(NH2)2 + OH- \rightleftharpoons OC(NH2)2 + HS-}$$

$$\ce{HS- + OH- \rightleftharpoons S^{2-} + H2O}$$

The pH of the solution should be higher unless all hydroxide is neutralized or consumed by the hydrolysis reaction.