Hi- I'm new to all this but I have some minor abilities to solder and follow a plan. I inherited what I think is an early SEX from my uncle and unfortunately the first thing I noticed is the sound is muffled and the midrange is recessed. I've played with a bunch of tubes and found the pair that sounds the best but the problem still persists. When listening, if I disconnect either the right or left input, I still hear sound in both sides of the earphones and the problem disappears, except of course that I'm listening to mono. Does this sound like a familiar problem to anyone? Thanks so much in advance for your help!