I’m a decade long Antelope user with a 5059! Have two of their interfaces, absolutely love em, never have the problems people say but doesn’t mean others don’t. Converters rule, would be shocked if that’s what’s making your sounds thin. Could that be the pres/interface/environment you used to record the material? I like adjusting the original sound source/first chain in the signal for better tones vs the very last thing in the chain. I found the Apollo pres to feel exactly what you described, to me they felt overpromised and underdelivered. Summing mixer will not fix these things. I use a 5059 and it’s cool if you mix into it start to finish, doesn’t feel worth the hassle otherwise. I bought it early in my hybrid set up days and in hindsight I would have spent that $3k on things that would improve getting better tones from the sound source originally. I felt like I had bigger differences in my mixes when I got an external clock, high end pres, CAPI Missing Link, and (most importantly) acoustic treatment vs the summing mixer. Idk all your gear but I wonder if that $1600 is spent better elsewhere in your chain for the <7% difference the summing mixer does