No one with any advice for me?

I have this Amp as standard with a little Pinkie Power supply. Headphones are Sennheiser HD 650.   I love the sound I am getting but it has been suggested that changing the tubes to Mullard CV4109 Would greatly improve the Sound Quality.   Advice would be appreciated please   Thanks

Friday on impulse bought a pair of HD 650's. This was after listening to the HD 558, 598, 600. Was hard to choose between the 600's and 650's but felt that the 650's were better for the type of music I listen to most. Jazz and Classical Guitar. These wil be replacing a pair of Grado SR60I...