I went over to the Sony center today to try the XBA 300, what you have mention is correct to an extent, I think the bass is a little weak, and I went futher and tried the XBA 300 with Sonys very own optional MU cord, with this the bass is slightly better, but still weak, im not the type that like extra bass, I just like them without modification, so I have all the EQ and stuff disabled.
For me using the XBA 3 with the ZX2 I have the volume about little over 51%, whiles my other iem EX600 and EX800 at 50%, for the XBA 300 i have to upper the volume to 65% to 70%, and i dont listen at high volume.
With the optional cable, the volume is slightly louder.
Overall Its a good iem, small size that will fit in most peoples ears, but you need custom cables, or sonys optional one to get better results, and may be the iem needs burning in, i should be getting a japanese version later on, this will be without the mic, so i'm sure the sound will be slightly louder.
Forgot to mention, the default cable is kind of thin, and looks cheap., whiles the main iem is very well built and looks cool.
On the side note, i still like the sound from dynamic drives more than BA.