We can’t escape the laws of Physics. Recorder is one of the earliest designs of flute with its reverse conical taper. I wasn’t a physics major, but where the finger hole is on an UNkeyed tenor is quieter in an uneven way from the other notes. If you move the hole down beyond a normalish kind of pinky reach, to where they place the hole but need the key to reach, it has a more even volume to the other notes. Frankly, I accept the lower volume as part of the recorder’s idio-voice (if that’s a word.) My Moeck Rosewood unkeyed tenor is a joy to play solo, though it’s lost when I play in a group. I have a keyed Plumwood Dream Edition (that cost a fortune) that I play with them. It really stands out.
Kenneth – Alto Recorder Lessons Project on YouTube