Best amp pairing with Aerial Acoustics Speakers?

" .But now with Theta being mostly Class D amps "

Not the Theta Dreadnaugt Mark 1 or 2. Traditional design and you can save a lot of money if you find a good used one. It's a multichannel amp so you would use 4 channels in a vertical biamp. Also, that amp was designed by Ayre, so it makes sense so recommend them as well. In my own system I do a vertical biamp with a pair of V-5's. That would be another good option, and you can start out with just 1 amp.

" Your speakers are not hard to drive on the top end but do present a challenge to drive on the low end. One solution is to horizontally bi-amp them using two different amps speced for their unique loads ."

That sounds like a good idea, but in practice using 2 different amps horizontal is nothing short of a train wreck. The only systems I've seen that type of setup work, was when someone was desperate to fix a problem. And even then, they should have just fixed it the right way.

" You could elaborate on why your experience is different. That would be more helpful to OP than just saying it is a train wreck. "

Sure. My experience is different because everytime I've done it the system sounded like crap.

" Yes, the problem is many speakers are actually two separate speakers in one enclosure. Hence, two different amps each doing what it is designed best to do. One handling the highs/mids and the other handling the lows."

What makes you think you know more than the designer? Aerial didn't get to where they are by making mediocre speakers that needed to be fixed with different types of amps.

" In audio the right way is what provides the most optimal results in a cost effective manner. Sure, OP could just go out and buy a TOTL Pass Labs and that would work. But I am trying to offer a solution that I have used in many configurations that can sound just a good if done right as a TOTL high end single amp .....at a fraction of the cost."

Then lets have it. Lay out a system of specific components that the OP can buy doing it your way.