It's impossible to say which of what's been mentioned would be "better". Most of the reputable diy kits are heads and shoulders above commercial offerings until you get to what I consider ludicrous price levels, so you're already ahead of the curve, whichever choice you make.
I have a more modest pair of Danny Richey designed speakers, the xls-encore, and they sound pretty wonderful considering their cost. Having never heard the AV/3, I can't say how they compare, but most of the designers tend to voice their speakers similarly. I have heard other Danny Richey designs at RMAF and such, and all were similarly easy on the ears.
But keep in mind how you plan to use them. Your room is large, you sit 16' away from the speakers, and you occasionally like to crank it up to 11. Any of the diysoundgroup speakers mentioned would be far more capable of such feats, and likely be able to do so with your existing AVR. Also, diysoundgroup does a fantastic job with the flat packs, the panels are all dado'd and go together perfectly, making for a novice-friendly build. There is probably more variability with regards to their voicing, as there are several designs from different folks on that site. All I can attest to personally are the Tempests, which Jeff Bagby voiced to be just slightly on the warm side of neutral, which I like and works perfectly in my situation.
And big-picture perspective, keep a second or third sub on your diy to-do list. I suspect that your large, square room would benefit handsomely from multiple subs.