The Venere 3 are great speakers for the price and should be quite competent down to at least 40hz. A sub like the Fathom, or a REL, should do a great job of filling in the low bass the Venere can’t reach. If you’re not getting mid-bass from the Venere, there’s something else going on. You may have a serious room mode problem or the Venere or listening chair need to be repositioned. Before you dial in the sub (crwilli’s post is spot-on for this), try positioning the speakers using the “master setup process” found in this thread:
https://www.audioaficionado.org/showthread.php?t=46634
Also if you have any way of measuring your room response, that might tell a story.
As for subs for mid-bass, it’s a bit of an oxymoron: a subwoofer is called that because it’s intended for sub-bass. I can’t think of a bookshelf or stand mount in any reasonable price range that doesn’t get down to 50-60hz. I’m guessing the only “subs” that cover mid-bass are those sold for desktop systems, soundbars, or all-in-one systems like Bluesound and Sonos.