Cool! I think these are good for the price.
Speaking of weak bass though, PM-1 and PM-2 has even weaker, less impact-ful bass.
Yet, some people like them so I guess YMMV.

Why would a more powerful amp do anything for the PM-3 considering how efficient it is? I have a PM-3 myself and I find that adding more power per se doesn't necessarily make it sound better.

Have you tried using a tube? It is very efficient as is no question about that but I found that when I hooked it up to my tube amp (Garage1217 Project Horizon 3 [150-300ohm]) it really opened itself up and gave me the warm bass response I really enjoy. Now when I paired it to my Fiio E11k Kilamanjaro 2 then the increase is very minimal but still somewhat notable. 

Adding to @Army-Firedawg , efficiency does not equal lack of ability to scale up with more power or better sounding amps.