Cwlondon,
I don't think there's any one way to upgrade the crossovers. The best results, as with any speaker, will be with bi- or tri-amping. Many advantages -- you can use the best components, the amplifier is always connected directly to the load and seeing a beautifully non-reactive impedance, you can select amps that are best for each frequency range. Of course, it's also the most elaborate approach.
In an unmodified IVa, I believe it would probably mean an active crossover rather than a PLLXO, since we're talking high order crossover slopes. Another possibility would be to use a digital crossover. That's what I'll be doing with my own IVa's, but all my sources are digital -- I know that some people want to stay in the analog domain. The best and most practical way to go digital is probably DEQX, but the crossover can also be PC-based.
Of course, the simplest approach is to upgrade the quality of the high-level components. Upgrading the caps seems to have a bigger payoff than upgrading the chokes, that at least is what some people say.