There is no phase noise difference when powering the DRIXO oscillator with 15 or 18V.
We have measured the phase noise with the TWRPS-UGL at 15V, with the TWRPS-pp at 15V and 18V, with the TWRPS-LBS-M battery at 16V5. Always the plots overlapped.
The TWRPS-UGL is designed to output 15V. In order to get 18V one or more zener diodes should be replaced. Not recommended.
15V was chosen because the phase noise of the DRIXO is exactly the same in respect to power it with 18V.
Bypassing the rectifiers is enough simple, then one can provide DC directly to the board.
The the raw supply has to be 18 to 20 VAC.
Suitable transformer is the Bel Signal DP-241-6-20 Mouser part