I assume it was the Bluetooth 5.0 support, which was implemented via a FW update:

I agree. I seem to remember I had some weird issue, and the 2.x firmware fixed it. I wouldn't even consider using an ES100 unless it had the firmware. With that said, if someone has damaged their es100 amplifier by plugging or unplugging it with the music playing (which generates a warning...

I own (2) ES100s, and I have had zero problems with the latest firmware. It's connection is as stable as a rock.

Ah, I understand. I'll measure my Believe and get you an exact measurements tomorrow :)

Since this is a direct replacement, the diameter would be the same diameter as the stock one installed on the Believe.

Glad you got it off. The glue must be stronger or weaker on some vs others. Mine came off without the need for heating. The volume knob was a fairly tight friction fit, but it did come off after working it off a little at a time.

When you open it up, the battery should say somewhere on it some numbers. These numbers translate to the mm dimensions. For example, if it says 3.7v 703030, that is a 3.7v battery that is 70mm x 30mm x 30mm. It may not say on the top, it may say under the double sided tape where they stock it...

Maybe the guy a few posts up that smashed his can give you the battery

Link?

Interesting! I've never noticed that. I'll take a look tonight :)

I have no experience with any of those. But I can say from my own gear, I noticed a definite difference between my xduoo X3, ES100 in balanced mode, and especially the FiiO A5.

What are you driving them with? The more powerful the source, the better.

It says fully balanced, but not sure on what size jack(s) it has.

Price?