That’s interesting regarding the VAC power supply fuse holder. I’m 100% sure I loaded from the top, since my preamp sits inside my entertainment center and can only be accessed from the top.
I found the M1 fuse in my N20 to provide a neutral sound and more definition and presence. I’m not sure of a “bump.” I don’t think that I’ll move to the M1 for the VAC due to the long burn in time. Have you tried the SR fuses? I haven’t tried the purples but have an orange in my Theta. They don’t seem to take as long to burn in, but they blow pretty easily.