Hi,
First thanks Amir for another great review!
Although this may not be the perfectly right place I feel I need to share some of my experience here concerning portable BT receivers, including the UP4. Mainly because below each of these reviews, a discussion about the alternatives starts at some point. And I briefly owned the BTR3K, UP4, ES100, BTR5 and finally settled on the Qudelix 5K. I am not an audiophile and actually just started to get into the technicalities of audio gear while searching for a way to do so wirelessly. I was perfectly content with my iPhone 6S's output before upgrading to an XS. I mainly listen to music while doing sports, outside and inside. I've got several wired headphones with different levels of noise isolation for these purposes that I like very much. And I didn't want to search all over again for wireless options when losing the headphone jack (found the lightning dongle kind of impractical). Plus I like the convenience of not having to throw away perfectly good headphones because the non-replaceable battery died.
That's when I discovered that there was such a thing as tiny Bluetooth receivers that would do the trick. But even though all the mentioned devices measure at worst OKish for my demands, I was quite disappointed with the first one I got - the BTR3. Turned out I couldn't tell a difference between iPhone 6S out and the BTR3 on USB, AptX (from my windows laptop) even SBC was fine. But AAC sounded more like playback from a cassette (source volume at max as recommended). Somewhat overly warm. Incidentally I was setting up a raspberry pi + dedicated digital out as an Airplay / Bluetooth sink for my old AV receiver at the time and thus knew that AAC was not supposed to sound THAT bad (at least not when using the Frauenhofer AAC implementation). Actually I couldn't tell the difference with the source being an already compressed Spotify stream on that setup. So the BTR3 went back and I got the UP4 instead. At first I was blown away by what I percieved as clarity and resolution. But I soon noticed that some songs I liked a lot were hard to listen to because the region around 4-7 kHz had an almost painfully piercing characteristic to it. Again, it turned out that over USB everything was fine. SBC, too. Well, signature-wise, anyways. I thought I could hear quality-degradation, which may or may not have been placebo. But AAC was a different story. In my layman's terms I would say soundstage somewhat narrowed and there was this uncomfortable, piercing quality to the upper midrange (?). I later saw at least one post in a different forum talking about distortion in the highs over BT with the UP4. Maybe that is it. By that point, I was kind of fed up with BT receivers already, but got the ES100. Finally, all codecs I could test sounded good. Different level of compression artefacts maybe but otherwise the same experience. I won't go into detail why I got the BTR5 and finally the Qudelix. The ES100 had quality issues with the charging port and the BTR5 had some firmware glitches and then the Qudelix was released and I loved the app of the ES100. Anyway, I couldn't tell a difference between wired and AAC with any of these last three and in particular even ABX'd the Qudelix versus the apparently-not-so-bad Apple Lightning adapter with a devastating (or perfect
50% correct identification just out of curiosity.
My bottom line is that while for my purposes all these devices should be fine as far as the predominantly wired measurements I can find go there are definitive differences in Codec implementation - at least AAC. I could not test LDAC or any other options except aptX or SBC. Nor do I have headphones with balanced cables (although I doubt that should make a difference). So even if the optimum use case is with higher grade codecs or USB, it would be quite helpful to see how these devices perform in their dedicated use case - as BT receivers, possibly with different codecs. Or at least bear in mind that despite good measurements, codec implementation might be off.
I must add that I could only test the BTR3 and UP4 for a couple of weeks (around June 2020) before the return window closing. So both companies might have fixed the issue with a firmware upgrade.
