mono block or bi-amps which would you pick?

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
  

Are speakers bi-ampable? Some speakers come with a provision for bi-amplification by offering bi-amp switching on the backside, this allows bypassing the internal passive crossover and will require external active crossover instead.
The main advantage of true bi-amplification is reduction of distortion caused by internal passive crossovers at high volume playback volumes.
A good stereo amp is just as good as two mono blocks, mono blocks make sense only if very high volume playback levels are needed, I would personally avoid then to reduce clutter and keep things a bit simple.
Another advantage of bi amplification is much reduced power requirement from each amp compared to conventional systems, this again is another good reason for choosing stereo amp instead of monoblocks.
I have  JBL 4350 speakers, these are designed strictly for bi-amplification, the hi and low ends are internally not connected and must be bi-amplified. I use two 200 watts stereo amps with active crossover, the low end is 96 dB and the high end is 99 dB. My music room is very large 18' x 27' with high sloping ceiling, the loudest sound level that I can tolerate requires 30 watts per channel max, I see absolutely no reason or justification for monoblocks with this setup, high quality stereo apps are more than adequate.