Sorry for posting a reply so late after you made your original post, but I was wondering if I could pick your brain on the 05BL, since you seems to be one of the few people who've bought it. The form factor is absolutely perfect for my use case and I’ve had my eye on it for the last couple days.
As far as I'm aware, both the 05BL and the ES100 use aptX and ACC codecs. The ES100 has the capability to do aptX HD with its CSR8675 chipset (the 05BL uses an older CSR8670), but this is only enabled for certain devices (namely excluding iPhones, which only use AAC). aptX is supposed to be equivalent to 16bit/44.1kHz, and aptX HD closer to 24bit/96kHz.
However, even with the aptX only chipset, I see nothing in any of the specifications of the hardware used in the 05BL that implies audio would be so bad that "lyrics were totally not understandable," so I'm a bit confused how that happened... It’s possible their implementation of the rest of the hardware was just really, really bad, but considering the hardware needed to go from bluetooth -> SPDIF is fairly basic (the CSR8670 SoC literally has an on board SPDIF output), I’m less inclined to believe that poor implementation could leave stuff unintelligible….
One thing I noticed is that you are using the line out of the ES100, bypassing the internal DAC. Is it possible you left the XD-05 on AUX IN mode when testing the 05BL? This would *likely* (i.e. if I was designing it, not based on any data) be using the internal DAC of the CSR8670 SoC, which is not designed to sound all that great since it’s usually never used.
If not, is it possible the material you tested with was different between the two devices? Especially if you were testing with an apple device, the codec is very dependent on the source material. AAC or lossless (FLAC/ALAC/WAV) are usually the formats that pass through aptX/AAC the best. Streamed material that isn’t in AAC or other audio formats like OGG or MP3 will end up passing over SBC, which absolutely wreaks the audio quality. If the material you were testing with was in these formats, this could also have been what happened.
Otherwise, if none of these apply, it might really be just a terrible implementation… the 05BL has such a perfect form factor that I really want it to work well, and I’d be pretty sad if I had to use a 3rd party solution…