Wow, it would be a dream to have a custom room.
I've often wished that when we were doing the renovations here, I'd expanded my listening room, which had a small bedroom built inside it in the 19th century. Now, it would be a huge messy project. About the only thing I did was run dedicated power lines and put in provisions for hiding cables, which I ended up not using anyway.
Know what you mean about the midbass. Which should be even better in the 30.7's than the IVa's because of the new 4-way midbass driver. Also, it's a brand new midrange that apparently brings the quality of the midrange in line with the midbass and ribbon tweeter performance.
I'm looking forward to the demo too, though Wendell says you'll hear really good sound in only about 20% of the dealers because a lot of them have acoustics issues. But I gather the dealers will make special listening arrangements for those who are interested in buying a pair.