I have had multiple S-1 and S-2 heads with transient responses that could
not be leveled. In all but one case, the problem was caused by R18 that
had drifted high by 100% - 150%. It also caused increased signal noise,
especially with the sampling head case removed. (All my sampling heads
still have original diodes, though.)
I thought I'd mention this, as it is quick and easy to check, and fairly easy to fix.