Dear Dr. Loewenheim   My husband and I would like to express our thanks for the wonderful concert by the Youth Philharmonic Orchestra on Sunday evening.  It was a marvelous treat!!   We appreciate all the hard work and long hours you, the members of the orchestra, and your support staff put into the performance.  When we looked at the program on the website earlier in the week it looked like an ambitious undertaking but it was beautifully executed. The quality of the sound, the poise of the performers, and the professional presentation were a joy to behold.   We thank you for taking over the position as conductor.  You obviously have the knack of bringing out the best in these young people and we look forward to the next performance.   With Best Wishes,   Liz and John Shields