Good for you to follow the Linkwitz route closely. Still, given the exchanges I had with SL about both Orion & 521 while he was still alive, I'm pretty sure he'd have been both sympathetic with my path as well as curious about the end results. He was very much a DIYer & never stopped tweaking his published designs.
And yes, you're right, the appearance of my speakers are low priority because they live in my studio/workshop as references. They are not intended for the LR, where the Orions are doing fine duty.
TBH, my original intent was to build both NaO Note II by John K. as well as the LX521. The 2 designs were so close I felt compelled to compare them. At the time I ordered, some components were out of stock at Solen. When I started building, I used the parts I did receive. The dipole AMT was the only component I planned from the start to veer from both sets of plans: it just made more sense than 2 dome tweeters per speaker, an approach I'd already used in the Linkwitz Orions.
I was not pleased with flex of the 18mm BB plywood top baffle. It ran counter to Linkwitz's own efforts to minimize vibration in the driver baffle of the Orion. The better supported NaO Note design seemed superior, but the 521 shape was more visually interesting to me, so I experimented with several variations for improved rigidity -- a 24mm BB and composites of BB+MDF and BB+aerolam+BB. Aerolam is the honeycomb aluminum + thin skin outer panels used for internal walls and floors of passenger plans. The 4x8 sheet of 12mm thick fiberglass skin aerolam was never used surplus from a YVR service company for Alaska Air that I acquired precisely for applications like this. You might know that B&W Celestion used aluminum skin aerolam for its celebrated SL600 and SL700 monitors.
The 1.2" thick BB+MDF baffles were the 3rd set I tried. They stayed on because I chickened out of using the aerolam composites. Cutting it created dangerous fine aluminum & fiberglass dust & shards that my dust extraction devices at the time didn't handle well.
I still intend to reduce the vibrations in the baffles. A magnet-mounting of the 8" driver, as used in the Orions, is in the works.
I'd love to try the Linkwitz 22MG, but the €290 price for each is sobering. Likely means >CA$1000 to land it here. Maybe if a friend wants an original up-to-date lx521. I haven't seen any tests or measurements of this driver -- do you have any to share? Is it a variation of the 8" MG driver used in the Orion??
I have no doubt the L26RO4Y04 is a more powerful woofer than the SBA SB29WNRX75 I used, but by the time it came back in stock at Solen, the price had jumped to nearly what it is today -- CA$750, or $3000 for 4 -- nearly 3x that of the SBA. I calculated that the primary difference would be max SPL, the Xmax difference of 15mm vs 11mm: 102 dB vs 98 dB at 30Hz. With a pair of drivers per speaker, the theoretical max is 114 dB vs 110 dB. I didn't think I'd miss the extra 4 dB. The last measurement I made of one 521 clone in a vary large room (around 80x40' with 15~20' ceiling) showed flat response at 30 Hz & around 5-6 dB at 25 Hz. This at 95 dB/1m.
My miniDSP 4x10 HD crossovers are LR4 at 120 & 1000 Hz, and LR4 at 3500 Hz. These were derived experimentally, but they are close to both NaO and 521 originals -- tried both sets with endless variations. In all, I spent some 3 years experimenting with upper drivers & crossover/PEQ settings before settling on the current config. I have a new Flex 8 that will replace the 4x10HD soon.