Many amps will work with HD650, but tube amps are by far the best for HD650. I'm trying to avoid buying just a tube amp for my headphones UNLESS IT CAN be used as a preamp also, then I'm fine with it. I dont want to end up with headphone amp, tube preamp, plus a regular amp. So its find a good tube headphone amp that has pre amp ability, or tube pre amp with regular amp with good headphone jack. . . . Headphone box im not sold on, she did reply to me and said I THINK ALL OHMS SHOULD WORK, not sounding totally sure. I not sold on it for some reason, never even heard of one or know anybody using one, that dont mean it dont work, guess im not convinced.
It certainly makes sense for you not to duplicate your efforts.
The specs for the HD650s say their impedance is 300 ohms. I would post the schematic of the adapter box on a headphone forum and ask for confirmation that it will work with a 300 ohm load. If it's not ideal as shown, I'm sure a couple of parts values can be changed to optimize it. An amp with a headphone jack will have similar circuitry in it, just not in a separate box. Her project just allows you to add that capability without buying a new amp.
You might also do some additional research on the design, which is not something that Stephe came up with, she just built it. A wider search will likely turn up other similar designs.
https://www.skunkiedesigns.com/headphone
If you DIY and use point to point there is no reason why you can't incorporate the headphone interface (whichever one is suitable) into the same chassis as a tube amp. And, if you only ever intend to use that one amp, you can incorporate the preamp section into the same chassis. This is what's known as an integrated amp.
Most tube amps include a preamp section although some may require an additional, external, preamp in order to drive the amp to full power output. Of course, full power means nothing when it comes to headphones because they need so little power. Half a watt (500mW) will probably give you permanent hearing damage.
And even with speakers there's no meaningful difference between, say 75% and full power in most real world situations. The difference is 1db of output with the amp turned up all the way. With most amps, under normal listening conditions, you don't run an amp at max output so the difference in volume is like what you'd hear if you leaned forward in your chair.
My computer room is 11x11, just barely smaller than the room you described earlier. I use USB from my computer to a DAC and then to my DIY Bugle 45 SET, which is only about 2wpc. The volume control on the amp is typically at around 9:00 to 10:00 and never higher than 12:00 for any extended time.
I built the Boogie Factor because I wanted a preamp that could be used with a variety of amps but the primary target was the Voice of Music SE 6BQ5 console amp whose original preamp section was connected by an umbilical. I didn't have the preamp part, only the amp. So I modded the amp so it could be used with any external preamp and built the Boogie Factor. I use the same amp with the little Nobsound E6. And, as I've explained, it can actually operate without any preamp, though it produces less power like that, probably similar to my Bugle (?).
That's the amp that Stephe built and uses to test preamps. Because there is no preamp tube on the amp chassis it lets you hear the external preamp without any influence from other tubes.
If you plan on using a variety of power amps you could use a separate preamp, Iike I do with the Boogie Factor and the E6, but if you only plan on using one amp everything you're looking for (preamp, amp with speaker outputs and headphone capability) could be built into a single chassis.
