Hi hmani,
The SPI Mode depends in how the chip works in sending the data via SPI. You can see more here http://en.wikipedia.org/wiki/Serial_Peripheral_Interface_Bus#Mode_numbers . I'm using the SPI mode 3, as it's the only one that gives the correct data for the registers - I tried to read the register 0x48 and it gave 0x3F, so it's right.
Now I'm using a new PCB with the ADE7758 less that 1cm away from the microcontroller, instead of the old PCB where I had it more than 6cm away. I'm using the 1MHz frequency for the SPI connection but I think maybe is still high as my microcontroller sometimes can't get the right value.
Regards,
breitas