OK, wading in here briefly. A few general pointers / thoughts, for what they're worth.
-Fostex's factory horn designs do not in general have a stellar reputation, but not all of them are bad, and the ESigma types are reasonable, have decent enough gain & are relatively linear for horns of this type. The standard En series horns (which were actually designed for the earlier E series drivers & have been carried over) are best avoided however. As an aside, in my experience, the E series & En drivers work better in the ESigma boxes than they do in the horns nominally designed for them. A point to keep in mind: they are designed very much with Japanese priorities in mind, with an eye toward room sizes, construction, and amplifiers -Fostex designed these units on the assumption they would be used on the end of a high output impedance amplifier (think ~ 2.5 ohms or > ).
-Re other drivers, it depends on which, specifically, you mean. You can use any driver that physically fits the enclosure, but unless it has identical specs. to the design driver, you will see significant deviations in response terms. How much so depends on degree. Unless the enclosure has been deliberately designed to be flexible enough to accommodate a modest range of driver specs., as a rule, it's better to avoid significant changes in spec., most notably Q, F0 (Fs) and Vas, which are the dominant factors.
