Great review and I find these kinds of posts very valuable. It's the same thing I experience with my Infinity RC263, R263 X4 setup. They are so darn good that even spending a lot more doesn't mean it woud get better. And as weird as it sounds, sometimes some speakers are a match for the room and in this case I think these infinity's just work for the living room I have them in.
I even tried replacing the RC263 with a Revel C208 and in the end, I valued the uniformity across all speakers and kept the RC263 and returned the C208.
In other cases, with a large theater room or something, I could easily see other speakers being better. But I've pretty much given up on trying to redo that room and just enjoy the bargain I got for that one.
Sounds like that's the case for the Monolith and I'd love you to get some of those THX's to compare because I had considered getting some of those myself if the price was right or I found a used set for the right price.