There is always a trade off..
General recipe for horns systems are cut from 500 to 800hz , you will need a tweeter at around 5k to 8 khz, and a 15 inch woofer from 500hz to 40hz. Depending an the drivers chosen.
I have tried a bunch of drivers horn loaded from 150-180hz Lowthers loose authority at 150hz but have a beautiful tone in a front horn, Fostex do get down to 150 hz nice but sound less dynamic and transparent than lowther
(I didnt try the F200, its in my list of things to do!)
The thing about this IMHO is that you get the whizzer highs... If you cut the Lowther at 1 or 2khz or higher over to a tweeter or a compression driver, then you fix that but you have a cut right there in the middle, which is worst? You decide based on listening tests. You can try the smaller Lowther...
You can get a GOTO driver specified from 100hz to 8khz, but GOTO recommends smaller freq. spans.
Or you can try a Phenolic Diaphragm Altec or Great Plains Audio compression driver that goes down to 300hz up to 5khz on the Azura 340 horn and stay away as posible from the human voice freq. range (this driver was designed for the human voice freq. range) Of course you will need a tweeter but the bass integration will be easier, then again you are cutting the compression driver too close to the freq. rate of the mouth and that induces colorations, which is worst? Lets try them.
A good compromise was found a long time ago with the general recipe, what I dont like about that is the tone of the 15 inch driver at 800hz, it sounds dull, that is why a lot of people are trying to get their compression drivers to go lower, maybe loosing definition in highs (depending on the driver) that is why I recommend a bass driver that goes up to 3khz, so that bass at 800hz has the same dynamic taste and detail as the compression driver and you dont get a "step" in performance...then we need to find a really sensitive bass driver, maybe two!