My advice:
get the AT's unmodded to accept phantom power, and get a UA-5. I think that's the way you'd notice the best improvement. If you have the mics phantom powered, you'll be able to handle a lot more frequencies at loud volumes before they clip.... the UA-5 will provide your phantom power and your analog to digital conversion, and go optical out into an NJB3.
Unmodded mics: prolly done for $100 or so? (someone on the board can prolly do this for you....)
UA-5: used for $250 maybe?
NJB3: might be able to get one for $150 or so?
The only other thing you'd need to do is get batteries for the UA-5, which can be done for about $40-$50.
I might be off on those prices, but I think they're pretty close.
That's my $.02 - please disregard if you intend to stealth tape.