The only two reasons I can see for having a HOB
filter, assuming you have adequate live rock, is to either run some type of chemical filtration or to have a seeded biological filter than you can put on a QT at a moment's notice. You should have adequate bacteria in the tank by now that the HOB
filter isn't doing anything for you there. It wouldn't hurt to remove it for a few months and see if it does anything. You can always put it back on if things don't change.
I wouldn't do too much at once though. Take the HOB
off if you want, but give it a few months before you make the next move like a fuge. That way, you'll know what exactly did it. If you do everything at once, and things improve, you won't really know what helped you - the elimination of the HOB
or the addition of the refugium.