Here's a thought - Use a 12V (switching) supply from the wall, then provide separate linear regulators on-board for each effects unit - this will help reduce power-supply coupled noise (switching supplies are noisy, hence adding a linear regulator for every load) . However there is still a shared ground rail then - are these devices using opamps with a mid-rail synthetic audio ground though?
Or, use 9V switcher, add good supply filtering to the two 9V devices and linear regulators for the others?