I just found this cheap tweak for BHK power amps(and probably the preamp too…). Get some mouse pads and place it on top of the amp as shown in the photo. I bought(AUD$10 for 2) some from amazon and they were rubber backed and quite substantial(weighed around 100g each), don’t use light mouse pads.
I started with two pads and have now gone up to four pads. The improvements are more clarity, less harshness when listening at higher volumes and better bass definition. I think the pads are damping out vibrations that affect the tubes.
Please share your finding if you do try this tweak.