@InvisibleInk Probably best if you go to the ifi site and read the manuals for both...
Signature -https://ifi-audio.com/wp-content/uploads/2020/10/micro-iDSD-Signature-User-Manual_1.0.pdf
Diablo -
https://ifi-audio.com/wp-content/uploads/2021/03/iDSD-Diablo-manual_Ver1.4.0.pdf
Signature- they list the Signature output for both connectors as 4100mw @ 16ohm (no mention of balanced power)
Diablo - they list the Diablo Balanced output for the two output connectors as 4980mw - 4.4mm Balanced and 6.3mm Single Ended as 2417mw @32ohm
I am a little surprised they didn't put a fully balance circuit in the Signature, but that would have cut into their sales of the Diablo for people who want/need the power of the fully balanced amp stage.... note that the specs show the output for the Signature are 16 ohm load and the Diablo are 32ohm load.... huge difference on output from each...
The biggest drawback via the Diablo is that it is designed for Headphones and its not perfect for IEMs.... due to the amount of power out of the Diablo even with sensitive Headphones can be an issue as its best to adjust the volume knob from around 10 oclock to 2 oclock range to keep the volume pot wipers out of the mis balanced area when the volume knob is at say 7 oclock position... (super low volume level)
By ifi putting the fully balance amp circuits in the Diablo also allowed the design to have LO in balanced config and they even have a 4.4mm Pentaconn to 2xXLR connectors on the back for feeding other equipment...
Both are nice DAC/AMP but just wanted to mention if you have headphones with balanced cables and your headphones will scale properly with more power then the Diablo might be the better match for your headphones...