Dual rail supply help

I'm building a stereo amp with two Hypex UcD400 modules and one Hypex SMPS400 for the supply.

If I'm putting a simple pre buffer inside the amp and a raspberry pi - I will need a simple dual rail supply up to +/- 15V for the dual NE5344 based buffer and a +5V supply for the raspberry Pi

I'm guessing to buy a separate transformer so the SMPS only has to supply the UcD400 amp modules, also I would like the Pi to be always on and only switched by the IEC inlet switch.

Where would I tap off the 5V supply? Would I wire the transformer AC to two rectifiers and have two completley separate supplies? Or would I rectify and then use two regulators for the dual rail and feed one regulator for the 5V

or could I simply step down the voltage of the +supply rail to provide 5V, this is my preferred option but I don't know enough to tell what negative effect it ma have

sorry if its a confusing post,

How about feeding the Pi from a 5V phone charger, preferably a name brand one?

For the 15v supply, there are various options, all of which could be isolated from the digital and amplifier supplies, which should eliminate the possibility of ground loops and stuff:

An isolated dc-dc converter; I'd prefer to use a higher voltage than necessary and follow it either linear regulators. This could run from the 5v supply.

Untapped transformer feeding a half-wave rectifier, then regulators. That demands more from the regulators, but adding some RC filtering to the unregulated side could eliminate any worries.

Centre-tapped or dual winding transformer and full-wave rectifier, etc.