The problem is that if you really want to improve the JIII to get it anywhere near an AN J -- JHaving recently heard the JII it really is a very poor sounding version of what AN is doing -- they look similar but that is where the difference ends "completely." Still there is I believe a way to get them "in the ballpark." The AN E Kit is "in the ballpark" but it has clearly audible problems and is kind of putting a ford escort engine in a Ferrari - it may look like a Ferrari but it ain't one. The guts are what makes the end product. After hearingt he Kit E which is very good for the money -- isn't a true Audio Note - and this was built by an Audio note rep who designs and has repaired all sorts of gear for 20 years so his final product would likely be miles better than most kit builders.
Anyhow - the JIII drivers seem pricey -- you can buy two J drivers directly from Audio Note. They are from SEAS originally They sell the drivers individually and you could probably get someone to put them in for you
http://www.audionote.co.uk/index_comp.htm
The problem is to get them to sound more like an Audio ntoe you have to replace the tweeter - use a better cabinet, replace the crossovers, replace the wiring, change the port size, take out all the internal stuffing that Snell used and ensure that the tweeter uses no ferro-fluid cooling.
So totally different drivers
different crossovers
different wiring
different cabinet materials and bracing
different internal deamping design and no stuffing (well different stuffing depending on the cabinet material
Different port design.
And if this model has the rear tweeter then that has to be disconnected.
And you need a corner for it to truly work its best.
Consider that the AN J has 25hz deeper bass response than the Snell J and has obviously a far more open extended treble band by about 6khz. The Snell also was not matched to very tight tolerances. Which is not slight on Snell since no one in the entire industry matches to as tight a tolerances as the AN's. The current AN's are also time and phase coherent.
Still the Snell should be able to be improved greatly. The SEAS drivers from AN would improve transient response and their low distortion characterists would make the resulting sound less "muddy" and faster and tighter sounding. Getting the Foster Tonnegan tweeters should make the midrange and treble "clearer" and give you and overall sensitivity gain. You can;t change the ports so the speaker will need to be free standing which creates some colouration in room but you'll be no worse off that you were before in this regard. And even free-standing they still have prodigious enough bass to compete with most floorstanders but won't have the bass weight or the pressurization -- but then again who knows until you try.
There are few speakers worthy of fixing up -- this is probably one of them -- if you can get them 80% of the way to an AN J/L then I'd run them up against any competitor's standmount that I have heard at any price. After hearing a 10 year old AN J/L directly against the B&W N805 everyone in the room just laughed.
If you decide to sell them -- let me know how much you want for them.