Thanks a lot for the reply!

I really like the IFI Zen, but it's USB input only, and while I haven't been able to find definitive proof, I think my PS4 (a B-chassis, which is now pretty old) wouldn't be able to output audio through the USB (optical only).
I found a post on Reddit by a user claiming that a firmware update to the Zen allows it to work with a PS4 pro, but then again the pro is a more modern, more expensive version, so it might not be the same.

Would it be possible to use a 3.5 mm plug to USB adapter (see https://www.amazon.com/dp/B08ZKLC3RF/ref=cm_sw_em_r_mt_dp_F6KSK5FGT1YKMT14WJZ7?_encoding=UTF8&psc=1) to output audio from my monitor's 3.5 mm jack to the Zen, and use an external power supply to actually power the amp? Until now I used to plug my headphones straight into the monitor, but of course that only works with low impedance cans...

Unfortunately, the JDS Atom isn't available on Amazon, and importing from the US by buying from the JDS website would almost double the cost (postage + customs + taxes), which would put me quite close to being over budget . I've recently been shafted by customs, and I really don't want to give them more of my money or time.

The K5 is slightly above budget... if there's no other options, then I guess I'll have to consider biting the bullet, but that's a last resort, for now at least.