mono block or bi-amps which would you pick?

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!

Mono is better, except in instances where it is not.

Mono is simply transmitting a single signal channel to one or more speakers. That isn't better or worse necessarily, but if you're a die hard Beatles fan or into "AM oldies" I can understand how some could consider it preferable. 

Zipost, it sounds like the biamp alternative you are asking about is what is referred to as a horizontal passive biamp configuration, meaning one stereo amplifier for the mid/hi sections of the speakers and another for the bass sections, without using an electronic crossover "ahead" of the amps.  While some audiophiles use such a configuration successfully, it has many potential pitfalls that can lead to unsatisfactory results, especially if the two stereo amplifiers are different models.  And if the two amps are identical models, a better way to biamp them would be in a vertical passive biamp configuration, meaning that each amp would power a single speaker, with one channel of the amp serving the mid/hi section of the speaker, and the other channel serving the bass section.

In contrast to a horizontal biamp configuration, a vertical configuration would allow you to use shorter speaker cables, as in the case of the monoblock alternative.  And perhaps more significantly, it would also mean that both channels of each amp are processing the same signal, which can potentially reduce inter-channel crosstalk effects within the amp.

That said, with a biamp approach you are of course paying for four channels of amplification, while if you purchase a pair of monoblocks you are paying for two channels of amplification.  So assuming comparable power ratings for the two alternatives (and keep in mind that the difference between 250 watts and 100 watts is only 4 db) it would seem that whatever amount of money you decide to spend on amplification stands a good chance of getting you better quality amplification in the case of the monoblock approach.

Good luck.  Regards,
-- Al