Flat DAC/AMP combo with coaxial in for use with Focusrite 18i20

As title says, looking for a amp that I can feed from the coaxial out of my 18i20 3rd gen to get a better headphone out for mixing (and listening) that has a bit more power. This way I keep all my audio routed through one ASIO device. Budget can be fairly flexible and im based in the UK.

Currently I have Sennheiser HD598, Fostex TR-80’s and looking to grab a few IEM’s and Shure srh1540’s (and maybe the 1840’s as well).

The Fiio K5 Pro comes to mind, decently priced and with coaxial in. Not sure how flat or transparent it is. I dont mind a little bit of colour if I know what its going to colour in a transparent way so I can keep that in mind when mixing.

Not too keen on stuff like Topping stuff after the L30 mess that’s happened recently so I’d like to go with a amp that has good reputation and service.

Any ideas or suggestions would be greatly appreciated.