Let me break it down like this: An audio interfaces will provide you access to - dac + amp , XLR mic output and amp , volume control for balanced output, now this balanced out will provide you with - less electrical noise and l krrr krr when changing volume on big ass speakers (even the good one’s will produce more noise in unbalanced source). Attaching a subwoofer etc.
In case of headphones, you get amp + dac to drive high O headphones , thus also allowing better eq and gains , precise control rather than windows running audio etc.
So you gotta start with that.. a decent audio interfaces/dac/amp so something like that.. not very expensive ones but the one that gives you enough options.
Now with speaker, depends upon your room and purpose.. like movies? And gaming? The usual surround sound 5.1 or the bar + sub if you have less space.. if mixing and editing, studio monitors are expensive.
Headphones and iem? There are lots of reviews for what’s good for the price, can’t say for the IEM’s but for cans/headphones, AKG K361 are very good and close to the reference without blowing up, the Beyerdynamic will last you longer but while being more expensive. The ATH 40X, 50X , mix between the AKG and Dt770 . So sound wise, AKG 361 then 50X/770 , build wise , the opposite.
If you want open back, get either Sennheiser 599s or something similar for them or HI-FIMAN open back. For cheaper option, Koss ksc75 have been praised a lot
So these are good reference points which shows that it’s not the money that will give you the best audio regardless of the money spent, but you need both without overspending on either to get a good audio.. unless you fall for “Audiophile” and “Studio Mixing” grade, if it’s the latter and you are a professional , i don’t need to tell you any of this.. so don’t fall for the “Audiophile” marketing.
So this is my 2 cents on basis audio. Hope you don’t mind the few mistakes I made while typing this on phone.
in your case (I’m also curious about what upgrade in speaker have you brought, the JBL’s 308? or the jbl subwoofer?) the , the focusrite has actually made difference, maybe less in terms of sound you hear, but more in terms of control over it and the more choices it allows you to connect.. like trs balanced.. maybe try high res Audi with precise eq (some digging up the settings required initially) , you will hear the difference if the output device is also good.