I'm going to start by disagreeing with the two previous reviews.   You DO NOT need to tube roll this amp.  The original Jan Philips 6922 are considered a premium version of the 6DJ8.  With the Sennheiser HD-600/HD-650s, you get what I'd call a more mild tube sound.  It's detailed enough to hear the page turns and periodic shuffling during some symphonic/chamber performance.   It's smooth, but not excessively, and has the punch you'd get with an SS amp.  Of course, you're probably buying this used at this point, so you may want to swap warn tubes...