Hi KTM_2018, :welcome:
I gather the Monoliths are still on their original panels, which will indeed likely have some high-end roll-off. And the Woofer is also original, so it will need to be replaced.
But first order of business is to put an active crossover on it. If you get a used DBX DriveRack 260, I can send you pre-made config files for the Monoliths.
You will need two amps, preferably with balanced line inputs, a ~200wpc solid state for the woofers, and a strong solid state amp for the panels, must be able to handle 1.5 Ohm impedance and highly reactive load, as you'll be driving the ESL directly through the Step-up transformer (that is still inside the cabinet of the Monolith) . I recommend no less than 400wpc into 8 ohms and rated at no less than 800wpc into 4. If the amp can't double the output with a halving of impedance, it need not apply.
Sunfire amps work great on the ESL element of the speaker as do some other class-G or class-H amps. The woofers can be driven by class-D or A/B amps no problem.
BlueJeans cable is my recommended source for Balanced cables. If you need an unbalanced to balanced transformer adapter to feed the crossover, I have a spare Jensen Transformer I could sell. I used it between my Meridian preamp and the DriveRack in the past. Current preamp is balanced outs.
Anyway, much more to be said, but you can learn a lot by reading my system overview and following the various links to other threads referenced there for all maner of Monolith mods and upgrades.
Cheers,
