Good Amp for Martin Logan Monolith II

Try to find a used Innersound ESL amp. These are made specifically to run electrostatic loudspeakers. You could by a new Sanders ESL amp, (same as Innersound, but updated).

The Aragon amps seemed like they were struggling because they were. I'm surprised they didn't blow up.
Audio Research M-300 monos work wonders on the Monoliths, but you could probably get by with 100 tube watts depending on the size of your room.

Don't see any of these for sale on audiogon. Any amps that are commonly available that will work?

Try a Behringer EP1500.I use one with CLSs with amazing success.Don't use a preamp-connect the source directly and use the pots on the EP1500,which are os surprisingly good quality.

Get a McIntosh Mc-2300 7270 or 7300 300/watts/ch 1500-2000 in excellent condition and will drive any impedance curve the Monoliths can through at it

ARC is also a great amp for the Monoliths but they will be pricier being tube amps with the same output

You may want to consider Bi-amping the Monoliths they do sound better with an electronic crossover configuration