mono block or bi-amps which would you pick?

Mono-blocks are easy to set up and work correctly.

Bi-amping is more difficult to accomplish correctly.

I use 4 mono blocks & it sounds enormous! Every element is significantly improved. 

Zipost, I would PM Ralph Karsten (atmasphere). He builds, in my opinion, one of the finest tube amps and preamps available. He regularly contributes to Agon forums and is a source of unbiased opinion.
I trust his opinions implicitly.
Bob

zipost, four Atma-Sphere M60’s would be fantastic with your horn speakers! But to bi-amp, you’ll need an outboard active line-level electronic crossover to replace the passive speaker-level one included with the speakers. The First Watt B4 is a fine, moderately-priced 2-way.

You do not need 100-250 watts to drive those speakers! The whole point of horns and high efficiency is to allow you to use better lower powered amps. I would be far less concerned about monoblocks versus bi-amping and more concerned with finding the right amp.  These are some nice looking very high quality monoblocks:

https://www.audiogon.com/listings/tube-highly-modified-2018-01-04-amplifiers

Also it looks like Odeon speakers were initially imported by Axxis Usa longtime importers of Air Tight tube amps from Japan. Presumably the two brands would work together well. From the listings:

https://www.audiogon.com/listings/tube-a-reference-tube-amplifier-2018-01-06-amplifiers

Awesome looking speakers btw I bet they sound fantastic! Good luck and have fun!