Originally Posted by Schwab

I would suspect that adjusting the gap would make a bigger difference than having paper in between your mouthpiece and reciever. Last year I played for months with a piece of paper stuck in the reciever because the horn played better with a slightly larger gap. When I had time, I had my mouthpiece cut for Reeves sleeves.

Originally Posted by rowuk

When designing a horn, one should have some idea which frequencies will be amplified. The lowest frequencies are dependent on the diameter of the bell - technically called the mouth. The lower the frequency, the bigger the mouth.
Horns amplify most at these low frequencies, so the larger the bell, the less high frequencies are amplified. This is why a piccolo bell is smaller than a regular Bb - we put the amplification where it does the most good for the player.

Originally Posted by rowuk

A sharper flare reflects more lower frequency energy back into the horn. This makes the instrument slottier, but less efficient at those frequencies.