https://www.axiomaudio.com/ep500-replacement-amplifier

At that price you could just buy a whole sub from the b stock store. Hopefully you can find a cheaper way to get what you need.

How old is the ep500? It’s v2 so probably at least 8-10 years old I’m guessing?

Another less compact option would be remove or disconnect the plate amp and power externally with something low cost. Plenty of reasonable ( for low frequencies) class d options out there. Could be fun.

M60ti Hafler9505 & JFET Pre,Axiom Transformer. M3 Marantz PM7200 Dual 606 Denon 2700 M2 Yammy RX595