Since purchasing the M3's I have been inclined (no, say it's not true!) to, shall we say, enhance the sound, which leads us to tweaks and upgrades. Here are few of mine, and if you feel inclined, share your own, as knowledge is power, power is sound, and sound is everything.
Keep in mind that once one becomes acclimated to a sound improvement a paradox can happen. That paradox can take the form of making your system more sensitive, which means you hear more, but when you hear MORE, you listen even more with the critical ear and so on ad nauseam.
First there was the speaker cable. I was happy with Morrow Audio sp4 cable, but an old pair of MIT MH750's were on Craigslist. The MIT's were more solid with superior timbral accuracy. I have since moved up to the MIT M1 with MORE of what it is we are on the quest for.
EL34's, I have tried most current manufactured brands, and bit the bullet and purchased some Sophia Electric EL34's. WOW! Across the board in spades sound improvement from bass through highs. Clarity, openness, fullness, etc. I won't be going back if I can help it. YES, they're expensive!
Now that I had the MIT cables, an older MIT AC1 power cable also fit the bill. Much more open, solid, timbral, etc. not jaw dropping but worth it.
Bybee Quantum Signal Enhancer: I think they were about $90 a piece and I bought two, liked the improvement so much I bought two more. Crystals on steroids, improved imaging, clarity, bass tightness and the more (you buy) the merrier. Great tweak!
Gaia II isoacoustic footers: Interesting in that I almost decided to get rid of them as the "leading edge transients" or attack of the first notes, sounded more muted, and the sound was a bit homogenized and I was disappointed. But then my tweaking instinct told me to decouple the bottom rubber type suction cup from the floor and I put in some Herbie's gel type footers under them and WTF! Huge improvement, dynamic, bass, clarity, openness. Strings that previously were good were now great (on certain recordings of course)
Past tweaks would include Synergistic PHT phono, again, crystals on steroids with huge improvement, also High Fidelity cables RCA adapters, another "tweak" I would not want to be without.
Also, need to mention my DIY Shakti Hallographs. I thought due to the open baffle sound that they would be superfluous, but this is another tweak I would not want to be without. Being able to adjust the soundstage's openness via the Hallographs is a HUGE plus, as sometimes I can find the M3's a bit "in your face" depending on the recording.
Of the above, biggest improvement would probably be the tube upgrade, followed by the Gaia II's.
Hope this may start your (aural) salivary glands going. Of course, YMMV as all sounds are system/person/relationship dependant.
One final mention of setting up the M3's to get your imaging and tonal balance spot on.