Hi again Bob!
My advice for right now is to stick with what you have. Now I know that some folks do have some distaste for those units, but I can tell you I have used line6 stuff extensively in the past. Including an interface (maybe the same one you are using).
Can you do better? Yes. Will it completely destroy the quality of your work and cause the issues you are currently outlining? not quite.
I think getting to the point where you can produce comparable loudness, and also good quality mixes that dont unnaturally reach out and grab you in any sort of bad way(like the harshness) is a way to start. Once you have the technique down and your ears figured out to be able to produce stuff that sounds relatively loud enough and is pleasing to listen to.... your game will naturally elevate as your gear does. The harshness problem can exist no matter the interface.
Especially since you say you are recording all in the box, and you have a smaller interface.... at minimum I have to imagine you are programming the drums and maybe other elements since you arent recording acoustic instruments. In that regard, the interface wouldnt be contributing to any harshness or lack of loudness in those programmed elements.
How do you EQ? I've been right where you are and still end up there sometimes. For me, it ends up being a byproduct of solo'ing instruments and EQ'ing til they sound great all by themselves, and then not really paying enough attention to how they fit in the mix.
Like for example if you are a drummer.. you focus your but off on the drums and trying to make those cymbals really shimmer and shine. You eq to really make the snare crack and the kick and toms slap through. When you are done working on the drums.... you end up with something you think you like, but its actually pretty 'hifi' sounding in the mix. Has too much bite from like 2k to 4k. Sounds great when you audition solo with no context. Its a little bitey in the context of a mix, but you feel like you dont want to undo those EQ moves cause you could have sworn that they sounded great lol.
Its kind of an ego thing where you have to relinquish yourself to the idea that everything has to sit in a place, rather than trying to highlight your favorite instrument in such a way that it really makes them stand out.
The frequency that makes things jump out and be heard.....are those same upper midrange frequencies that in high doses can also make things sound shrill and given listener fatigue.
another problem frequency is somewhere between 350 and 500......even as high as 800 HZ. It can make things sound a little tubby and round into nasaly around 800 for lack of a better term.
Its common practice to scoop here a little to clean up the instruments. When you watch people EQ and what not, its like they almost always treat here a little. So you probably naturally adopt it.
the problem with digitally created, and often digitally recorded stuff.... is that it usually is NOT so thick and round in this area.
You pull those frequencies because its habit and you just think you need to in order to get that midrange mud out of the way, but in reality... the digital instruments and digitally recorded instruments already dont have much presence here.
In effect what you end up doing is reducing too much body. And at that point it is addition by subtraction.
You make the upper mids bite too much and sound shrill because you have taken away too much low mid.
Dont take any of this personally. Like I said I have direct experience with sabotaging my own stuff doing some of the same things lol
if you go back through the mix and find that you are adding a lot in the 2-4khz area to a lot of intruments, and removing a lot between 300-800 hz.... cut your eq moves in half. If you added 3 db of 2khz.. change it to adding 1.5 db
if you subtracted 3db at 350 hz, change it to subtracting 1.5
that alone will round things out.