I added an I-beam nylon string pickup system(passive) to my Alvarez classical guitar. I installed this myself, and tested it through my Marshall AS50D amp. It works great-but whenever I have tried to use the guitar in someone else's amp/PA set-up, it doesn't work. I've tried this on about 3-4 other amp set-ups, and it has never worked on any of them. Yet, it always works on my set-up. I'd rather not continue to bring the amp along whenever I play with other players. Any ideas?