I went through Heathrow last week and managed to freak out 2 security guards at the luggage screening. As soon as I put it my CMoy in the luggage tray for screening extra guards were called over because they were really concerned about this weird electronic gadget with a 9v battery in an Altoids tin.
It took a lot of explaining what a portable headphone amp was and why I needed it. This was also made harder because I was using the amp with Yuin PK1s which most people have a hard time believing are fairly expensive equipment judging from their budget appearance.
Eventually they let me through after confiscating the 9v battery, but after that harrowing experience I made sure to pack everything in my suitcase for the return journey. Anyway just wanted to see if this is a common experience for others.