I think in the two-sizes and 1/2 jacks part you missed the part where it'd be a single ~1KW miner. Within the context of this discussion there is no small miner and there is no miner that 2 jacks wouldn't immediately burst into flames on - unless you mean per board with a multi-board miner, but even then 3 jacks would be better for ~500W.
Quiet is relative, and also dependent heavily on the power consumption of the user setpoint as allowed in your second point.
If we're going USB, is it better to have an "internal daughter board" per machine that controls all internal hashboards, or relegate all board-level control to a microcontroller on the board and all boards connect to USB directly (either with an exposed port per board, or to an internal hub with single outgoing port)? I like the idea of direct USB connection to board-level control with no daughter board since, as you say, it removes one part from the list of minor things that can fail and take out the whole machine. One thing about direct control to boards is fan control - with multiple boards in a miner, which one drives the fan? What happens to the other boards if the fan-driving hashboard kicks out? It would be easy to have a jumper or DIP switch per board to designate as the "master", or just have each one attempt to kick on a fan and watch for tach response. It could also be possible to put a small controller on whatever internal hub is present, that controls fans for the whole machine, but at that point you're only one step away from the Avalon6 daughter board anyway. Maybe it's best to have a microcontroller on the board handling most things (convolving sensor readings, handling ASIC IO, setting voltage and measuring power use), but talking to a basic daughter board that multiplexes comms to all boards and also is a master control for the fan. This board needs to be simple and resilient, and ideally not requiring an FPGA to do its job.
I read size and power as per gh/ watt don't ask me why.
so if it is to be 16 by 9 by 9 and pull 1 kwatt dc as max.
2 boards with 3 pcie jacks each is a must.
I would like what ever connects the 2 boards to the controller be replaceable with ease
I would like a rasp pi or pc option.
When I ran 120 usb sticks to 1 pc I got it to be stable.
The good thing was every part in that was easy to replace.
Your setup needs every part to be easy to replace. via amazon newegg
The most expensive part would be the board and basically it the only part that would be yours and yours alone.Just like the usb sticks I only need to replace a usb stick which is your exclusive part.
So the key is to make the board a black box concept which the miner mails to you and you send a replacement.
and the board are independent so if a board dies it is one board. or ½ the hash
quiet is still key even if it must be down clocked.
at 500 watts it is more efficient and quiet.
at 1000 watts it is louder and less efficient.
to be frank I do think that a 400 watt to 800 watt is better then 500 to 1000
as so many good 850 to 1000 watt psus are around
https://www.amazon.com/EVGA-SuperNOVA-PLATINUM-Crossfire-220-PS-1000-V1/dp/B00SOXNK52/ref=sr_1_9?https://www.amazon.com/EVGA-SuperNOVA-TITANIUM-Warranty-220-T2-1000-X1/dp/B018JYHGQE/ref=sr_1_11_m?this would be really good if you had a 400 to 800 watt miner just clock the miner to 700 watts
https://www.amazon.com/SilverStone-Technology-Strider-Titanium-PS-ST80F-TI/dp/B01CE7NV84/ref=sr_1_1?and if the miner was on quiet mod at 400 watts in an office even this psu works
https://www.amazon.com/SilverStone-Technology-Strider-Titanium-PS-ST80F-TI/dp/B01CE7NV84/ref=sr_1_1?