The preamp has a low gain of about 12, there is only one gain stage while the other is a cathode follower. I do notice it does not have a input filter cap for high freq (1M) so I add C1 C2 100p each for you, to see if it helps. If there is any oscillation you can hear cracking sound from speaker as you move the tubes a bit. You really need a scope to see it. You can start with a point for VR1 and VR2 say 3V instead of 7V, 2sk373 is near constant current sink.
