Updating the above... I recently got an HD600 to try in my work system which includes GO2A and Vali 2 with Philips PCC88. As Marvey stated elsewhere, the HD600 is a much better match with Vali 2. The HD600 has a much less fatiguing sound than the HE400S in this setup. I now hear the "veil" that the HD650 had with Vali 2. The HD600 doesn't have this which is well known. Time to sell the HE400S... @imackler I suspect you should try a different tube with Vali 2 & HD600. I am going to find an EH 6CG7 to try, please let me know your thoughts if you already tried.