Hi Penance, Mullard tubes are apparently a very well recommended tube for the X-Can V2, and are apparently about £30 a pair. The stock Philips tubes are somewhat bright and forward, and Mullard tubes apparently give the sound a more rich and detailed feel. Valve amps are said to be the best match for the Sony headphones I've got, hence my decision to go for the X-Can as a cheap, tweakable and surprisingly potent (particularly when modded) amplifier. Now, where is WM with his suggestions... :D