To close that midrange gap you would have to change the Foster tweeter for a larger horn with a good compression driver or a 5 or 6" mid driver and tweeter .I modded my old SB112s with a Peavey horn and RX22 drivers and a Peavey crossover and my band has been using them for our PA for a few years now so there is room with that cabinet for a decent horn . Here's a pic of my cabinets . They really sound good with the Peavey subs we have for small to medium gigs .
If your cab is 8 ohm you can use one of the Eminence pre-built crossovers with decent results . I used a B&C ME10 horn with a Selenium D220ti OMF-8 driver and an Eminence PXB2:1K6 1600hz crossover in my 8 ohm 310 cab and have had great results with it . With the Avatar you would want to change out the horn attenuator for a 100 watt one as well . The parts would run around $100 and you would be able to use it for PA also .All these parts can be bought from Parts Express .
This mod won't get you a cabinet that sounds as smooth or good as what Duke is doing but it is a very big improvement over the standard Avatar mid-high end .
http://www.parts-express.com/pe/show...number=290-634 http://www.parts-express.com/pe/psho...number=264-270 http://www.parts-express.com/pe/show...number=294-618 http://www.parts-express.com/pe/show...number=260-262
Of course the standard Avatar crossover would have to be scrapped and the new crossover screwed to the back of the cabinet , the opening for the horn enlarged and the whole cabinet rewired .Here are a couple of other drivers that would probably work in this cab too .
http://www.parts-express.com/pe/show...number=290-442 http://www.parts-express.com/pe/show...number=290-526
They are both about the same driver but one is a neo so it's more expensive . Both would be a little smoother with less high end extension .
