Here is a start. Look and see, take lots of pics inside your speakers. I'd look at both crossovers.
Do a good inspection on the surround too. They are one of the main reasons they loose their lower end response. Easy to change and Infinity factory surrounds have always been available at reasonable prices too. I've used a lot through the years.
RS, Kappa and Reference series speakers all have some great speakers in their line ups. I've worked on a lot of Infinity speakers. Usually because of surrounds or a blown HF tweeter.
When I've worked a crossover I pay attention. They use some pretty good caps in some of those builds. Don't throw the baby out with the bath water. They can drift but for the most part unless it's an electrolytic it will probably hold up a long long time.. The inductors, resistors, diodes (2) you might want to look at and the fuse circuit always need attention. I don't see any LPads. Swap or add new red copper binding post. I'd split the circuit and have a bi-amp option.
I don't like valve amps doing bass duty.. AND ribbons deserve a valve amp. It's just night and day in how they sound.
I'd treat the cabinet too. Use Black Hole, SR71, Soundcoat..
Change the cheap white pillow batting for heavy course fiberglass in the bottom 2/3 of the cabinet.