You are wrong, the 3.5mm is a lackluster even on the NW-WM1Z. If you go for a Sony DAP, you've got to go for/invest in balanced cables.
The power output alone on single ended (60mW@16ohms) is a very good tell, that is very weak, not NW-ZX2 or NW-A35 weak, but weak enough that you can't drive any remotely demanding headphones on it.
To be honest the extra 10mW you get from the WM1 series aren't going to make a whole world of difference. Remember, that's not a typo, those are mW into 16 (yes, sixteen) ohms, as a reference, if you look at the hiby R6 Pro's SE output, that's 245mW@32Ohms, this is roughly twice the power you get on the WM1 series balanced output and over 8 times the power or the WM1 series SE.
While the balanced power output is somewhat decent on the WM1 and ZX300 series it's still somewhat on the weak side compared to its competition, for example, yet again looking at the R6 Pro, it sports a 750mW@32Ohms power output, I will let you do the maths as to how that compares to the 250mW@16ohms of the NW-WM1Z. That said I don't think anything needs that much power, but it's nice that it's there and you have the option to drive insanely high power hungry headphones, should those ever appear (I don't see anything requiring that much power other than electrostatics and those use their own amps/energizers), although then again we see plenty of desktop amps with over 1W of output, I guess some people just like buying gear with high numbers.