Sounds like parasitic oscillation, be sure the wires to the grids as short as possible. Use a chop stick to push the leads around and see if the problem can be made better (or worse). Depending on what power tube you are using and how efficient your speaker(s) is/are, the volume range can be pretty broad. The 6V6 is only about 4 watts while the EL34 could be more like 10-11 watts. Funny that you mention the 5E3, my first motivation to build a 2Stroke was because my 5E3 amps (when adjusted to their sweet spot) were too loud for my application. That’s before I knew what a great amp the 2Stroke is. BTW. my 2strokes are both pretty loud. I use them full up (they sound great that way) all the time and adjust the volume and distortion with the guitar volume control. If shortening the grid wires doesn’t fix the issue, post a gut photo or two and we’ll see if we can spot a problem area.