Hi Merlin,
I joined to the Asus router owners club four months ago, with a AC66U and using your firmware builds from 270.26b without any mystic issue. I'm happy with my AC66U, especially from 374_32 (it's easy because I have no any i device and I not use the 5Ghz and AiCloud).
Two weeks ago I offered a friend that I setting up and fine tuning his new AC56U. His key points the well working DLNA service and smooth, gapless DLNA streaming performance (fortunately he prefer to use wired connection). Under the setup process I faced some issue, I'm not sure that these issues are under your development/debugging focus, but maybe you can help.
First issue, when I connect a usb disk to AC56U with the following partition configurations the router reboot itself. ntfs+ext3 (USB2 enclosure) or fat32+ext3 (USB3 enclosure) partitions (both disk are 500GB, first big partition ntfs or fat32 and a small ~1.9GB second ext3 partition) When the router start to try access to disk I see that the accessing LED on the enclosure blinking 1 sec and after 1-2 sec the router restarting (if I don't remove the enclosure the router restarting under the boot process when initialized the usb port and start accessing the disk). Same effect with your 374.32, 374.33b1, 374.33b3 and stock 374.134, 374.158. With AC66U no any issue, I see both partition, on both disk in the web ui. Another reason (DLNA issue) I tried your 372_30_2 on AC56U and I was surprised that the router don't reboot with ntfs+fat32 disk and I see both partition in web ui. For temporally fix I delete the ext3 partition from fat32 disk (that I need to use with AC56U), I don't want to use with old 372_30_2 firmware.
The another issue, the traffic monitor show the wired traffic (in/out) under the internet traffic, added wired amount to internet amount (Outgoing packets to wired network to Outgoing Internet packets, Incoming packets from wired network to Incoming Internet packets) on Real-time, Last 24 hours, Daily, Monthly page. I see some small 1-3 pixel high spike on wired graph, but I could not pair with any event. Same affect with all firmware above. On AC66U with same router config no issue.
Thanks in advance
p.s.: sorry for poor English
p.s.2: I clear nvram before and after all firmware change and after reset factory default on admin page and just after then setting up the router manually (never load back config that saved different firmware version)