My German is nicht, but... One of the benefits of DIY is that you can get silly with enclosure size and shape because you don't have to ship it anywhere, and packaging / warehousing / shipping represent a massive amount of consumer product cost. Think of it as a larger bookshelf speaker pair with 4 18" (I think sealed, but again, nein sprechen) subs and it's not that unusually sized or excessive compared to a lot of AV buff setups. There are, of course, a lot of practical advantages to building that in 6 separate boxes, but if you're going to DIY, those are choices you get to make.
That being said, I would be very surprised if the handoff between the AMT tweeter and a 10" midbass went gracefully.