ANY IDEA WHAT MANUFACTURER?
I emailed and asked EMP about that.
Their response was worded very politely, but the bottom line is they "ain't sayin!"
That is really what you should expect. For example here is a
fictitious scenario:
Say Roger B Hassing happened to meet and befriend Frank Van Alstine, Frank becomes intrigued with designing a budget amp (after all, he never
gets to in his real life

). Then Roger finds some unheralded company in China to produce it.
Now, Frank doesn't want his name on it, because he has no control over the manufacturing quality and it may erode his reputation. Roger doesn't want the manufacturer's name to be known because they are unknown or they may have problems and he might switch production to another facility (which is not something you would want to advertise if you can catch the problems internally and make the switch smoothly).
Do you know who does EMP's subwoofer amps? They may be the same company as makes this tube amp.