I haven an ZX300 here and after MrWalkman constantly annoyed me about that, i installed Walkman One Firmware (i wanted to sell it but nobody is offering an acceptable price so why not just play around with it and it seems that it is easy to revert).
It is nowhere close to the WM1Z (Stock). Absolutely not in no single aspect. They share the same sound Signature but thats pretty much it.
The first thing that is the most obvious change is that the ZX300 sounds much more muffled and much more narrow.
If you don't need/use 4,4mm, even the ZX2 is an upgrade to the ZX300 and sounds better in almost all aspects. And i mean the 3,5mm from the ZX2 sounds better than the 4,4mm from the ZX300. The 3,5mm of the ZX300 is even worse.
Don't get me wrong, the ZX300 is an excellent DAP and worth its money, It sounds great for what it is, but it is nowhere close to the WM1Z and there is a reason why an used ZX2 is still more expensive than an ZX300.
This is the ZX300 board with its 4 FTCAP and tiny POSCAP+filmcapacitors and tiny LCs.
This is the WM1 board with its 13 FTCAP, OS-CON and large LCs. How an earth are they supposed to sound the same?
For comparison, this here is the amp part of the 3,5mm only ZX2 where you can see the smaller than WM1 but still larger than ZX300 LCs and the 7 OS-CON. The ZX2 3,5mm only output is more complex than the ZX300 4,4mm+3,5mm combined!
It is cool that you like the Firmware and that you like the ZX300, but please keep in mind that there are a lot of gullible people are here in this board who believe pretty much everything, including that software can replace hardware. They can not judge if you're telling the truth or if you're making jokes and they will think, that the ZX300 with Walkman One Firmware will sound like an WM1Z and spread the word. They will showcase their ZX300 to people with actual experience and knowledge and they will think "Holy crap, the WM1Z sounds that bad? (for its price)" not knowing, that they are fooled and the WM1Z sounds far from an ZX300 with firmware.
You can not replace Hardware with Software. It is impossible, it will always be impossible. That is not how physics work. Please stop giving wrong information to people who are seeking advice.
If you think this Firmware improves the sound of your ZX300, that is fine, tell the people that to your personal opinion, the ZX300 with custom firmware sounds better. But please don't tell people that it sounds like the WM1Z, thats just not true. The WM1Z, WM1A and even the ZX2 (if 3,5mm only) are still a noticeable upgrade form the ZX300. The ZX2 does have a slightly different sound signature though, i just threw it in as an example on how stripped down the ZX300 is.