DC voltage regulation
The TDA7294 (datasheet) will work fine with a power supply of +/-34 V.

A feature in its description is


  Thanks to the wide voltage range and to the high out current capability it is able to supply the highest power into both 4Ω and 8Ω loads
  even in presence of poor supply regulation, with high Supply Voltage Rejection.


This is encouraging, because it reads as if an ordinary audio amp power supply will be fine for it, i.e. no voltage regulators are needed.

Looking around the internet, it appears that you can add some film-type capacitors to the secondary side of the transformer to get a slightly cleaner supply:






simulate this circuit &ndash; Schematic created using CircuitLab

(If you put any filter capacitors on the mains side of the transformer then they must have the appropriate X rating.)

For a transformer, you will need a "timed" fuse rather than one which blows quickly because there can be a short but large inrush current when power is applied to it. For a toroidal transformer, you may want to be a little bit more generous with the current rating as the inrush current can be higher than for a laminated core one.

You could use two 6800 μF capacitors in parallel in place of each 10000 μF capacitor if makes it easier to fit into the available space, especially if it means you can use ones with a 63 V rating instead of 50 V.

Summary You do not need to change the voltage from your rectified-and-smoothed supply.