Tweaking a SRM-1 MK2 pp

[dukja]

The SRM-1 MK2 pp (rev. C, spritzer modded to balanced input) was having bad resistors when I just got it. The bad resistors are those 2.2Mohm (R303 & R304) resistors. After fixing that, I like this SMR-1's sound even better than my new SRM-323S for some music. The bass is tight and impactful and the soundstage is more open.

Recently, I changed the two diodes (D301 & D302) in the bias circuit with 1n4007. I also replace other two diodes (D101 & D201) in the L&R channels. I found the SQ actually went down (to my ear). The bass impact was weaker and the soundstage shrank. And the DC output from PSU seems to drop a few volts.

Then I put the old D301 & D302 back, the bass impact went back. The bias voltage I measured was about 450V before those 2.2Mohm resistors (as it is shown in the photo). I wonder why those changes and appreciate any suggestion on further improvement.