Diablo also does not have a Line Input, only Output to be used with other Amps.
The internal DACs in Signature and Diablo are probably better than almost all but the TOTL DAPs and even then you'll only get slight improvements or change in overall sound signature so it's not a big deal IMO.
If you are just looking for an Amp then Topping NX7 or xCAN are good in-expensive options.
Diablo does in fact have a line in, it is entirely possible and safe via the rear 4.4mm port. I’ve been listening with my Diablo with a line input from my M11 Plus using a five wire iFi 4.4mm to 4.4mm interconnect for the last several days.
Upon asking questions about it because I’d seen references this, the designer of the Diablo reached out on a private message and explained that the Diablo was in fact designed that way and with no USB data being input the DAC basically opens the gate between the port and the amp.
With the right cable you can use a balanced line in or a single ended line in. To go single ended the positives are used, the negatives are unconnected and the grounds connect. I made a 4.4mm to 2 x RCA cable to test this and tried it with a Modi Multibit DAC, it worked perfectly.
Have a look at the latter part of the Diablo thread. It’d be interesting to know if the Signature also allows this. I suspect not since apparently this was possible courtesy of later components in the Diablo that allowed the open gate past the DAC with no data.