Hey Martinman,
So your digital sounds poor and vinyl sounds good.Common problem! Lots of good advice so far. Here's my 2 cents.
Consider looking at your system as a whole...source, amp, speakers, room.There may be more to this than just improving the digital source and the fix may be easier than you think.
I think the fact that you don’t hear much difference in DACs is testimony to the fact that your source, the iphone, is bottlenecking your sound. But that many not be the entire or even major reason for the brightness/hardness. I have heard the Kef LS 50 speakers and they have wonderful detail but have a fair bit of high frequency output and not a lot of really low bass, they themselves are quite bright, especially if your room has little to absorb all that treble. You did not post what your vinyl playback gear is and that would have been useful.
I wonder if your phono cartridge might be a smooth sounding moving magnet cartridge. Some moving magnet cartridges have a bit of a mid bass hump and a bit rolled off highs and this would work in synergy with the Kefs, while the flat frequency response of your digital does not. That may be why you enjoy your vinyl set up more than your digital.
If that’s the case consider some or all of the following, changing your speakers, changing the speaker placement to damp and absorb the highs more, and/or inserting some tone controls like a Schitt Loki. The Loki is quite transparent, inexpensive and useful to adjust for these type of frequency abnormalities. You could place it in line between the DAC and Amplifier.
I would agree with daledee1 that getting a streamer is likely also a good upgrade but might be secondary to what I have already posted above.
With entry level streamers you have 2 reasonable options recommended in previous posts, Bluesound and IFi. Considering that the Bluesound node is now in it’s 3rd generation, there are lots of 2nd gen Node 2i models available on the market for cheap. When you get a streamer you also marry into the operating system and the BlueOs system is pretty user friendly and from what I have read more so than the the IFi. The IFi reportedly sounds better but has no DAC and will be dependant on that.
That being said, both devices will do direct, where you use the actual Tidal/Spotify app to play music. If you have no stored music files, their operating systems are not as relevant.
I own the Node 2i. First used it with it’s inboard DAC and then as a streamer only with an R2R Dac. The internal DAC, though not great in quality, is reasonable, and is not bright and glassy. The DAC upgrade I did gave me better detail, naturalness, and imaging BUT the tonal balance did not change much. I have since upgraded my streamer and my new streamer is with an R2R dac in my main system and the Bluesound node is back to full time duty in my second system.
I’m all for supporting brick and mortar hifi shops but so few of them allow home try out these days or you many not live near any. If you buy new, consider buying a Node or IFi from Amazon. That way you could try it and if it doesn’t help with your issue it is an easy return.The Schitt Loki tone controls also have a 15 day free trial.
Sorry for the long post....been into audio for about 55 years.... learned a thing or 2 along the way....hope it helps. This is 2022, digital has come a long way and you should be able to get very agreeable sound out of either digital or vinyl.