I recently added BOTH a pair of Fostex 6301Bs and a mono Avantone cube to my main setup (Focals, Events, Mackies, Tannoys).

The mono Avantone is mainly for checking balances and also making sure midrange instruments and vocals aren't completely stepping all over each other. The Fostex 6301Bs (in stereo) are a lo-fi reference that are limited to 80Hz-13kHz... Great for checking the "meat" of your mixes for real-world translation.

If things are getting too bright and crispy in the highs, or too carved out in the mids, or too thick in the upper lows, the 6301Bs will help reveal that. If the overall balances are wrong, the midrange is getting smeared, or the FX levels are not right, the Avantone will help sort all that out.

They both have their different and useful functions when mixing, alongside the hi-def monitors. Like you've most likely read before, get your sounds together on your main monitors; check and tweak on the mono cube and lo-fi references; check again on the mains. Repeat, as necessary.