Preamp vs Av receiver preouts

So I have a pretty good setup going on for myself, but my weak point is still my receiver on the music side of things (I use it for HT but I'm talking stereo right now). I have the Martin Logan ESLs, a 15" rythmik sub, yamaha aventage rxa-1060, Anthem mca225 hooked to the preouts of the yamaha (and a splitter to connect the sub L/R), and a schiit modi+. Would a separate preamp make much difference instead of the preouts on the receiver even though I use the yamaha in pure direct mode or is it not worth the addition? And if so what ballpark preamp are we talking to make said difference? Something like the parasound zpre3 looked interesting to me, but I haven't heard a separate preamp in a system before and don't know what difference it would make. I can't do much more in terms of room treatment in my room either. Thanks in advance!