I've heard a few Amaranthe songs. Pop metal is an amazing idea, but I don't like their style. The songs are littered with unnecessary double basses, aggressive guitar riffing and death metal vocals. They have this wannabe badass attitude vibe like too many metal bands, trying to look and sound tough... Damn shame. Thanks for the suggestion though.
I think scattered over a his last few albums, Devin Townsend made a couple of way superior radio friendly pop/metal/popmetal songs. Ih-Ah!, Juno, Where We Belong, Save Our Now, Heatwave, Fallout, Silent Militia, among others...
Aside from that, I think Alestorm is the closest to party music I know in metal. Hangover is a pop cover, and Magnetic North apparantly is a deliberate Lady Gaga ripoff just so Bowes could make this mindblowing mashup video:
https://www.youtube.com/watch?v=JKF2oc9h3o4So yeah, there's pop metal to be found here and there. Synth pop metal though? Much rarer it seems.