Most Effective Tweaks?

Yep, as mentioned several times above, hum is most likely ground and/or EM related due to close proximity to transformers and the ilk.

RF is typically higher frequency noise, not unlike the noise other than hiss one might hear listening to FM stereo radio. Older digital gear, microwaves, light dimmers are just some of the more common culprits.

I realize this will probably come across as argumentative or beating a dead donkey, nevertheless what I’m actually referring to is not EM or RF. I’m referring to the induced Magnetic field from the transformer or in some cases from adjacent AC cords. That’s why wrapping mu metal around a transformer improves the sound, because the mu metal absorbs the magnetic field. Mu metal has nada to do with shielding RF or any such thing. It’s also why it’s a good idea to avoid parallel runs of any cables or cords. In some cases the 60 Hz hum is due to the physical mechanical vibration of the transformer.

Most Effective Tweaks?

Fuses are the most effective tweaks.....   price/gain ratio  is by far the best. 

the music you play #1
room acoustics positioning  and treatment DYI mostly.  transformational
speaker cables significant
speaker stands, points,  proper coupling and isolation (I use both right now) significant to transformational
Green pen the edges of your CD's center and outer edges (very significant to transformational) $3.00
Acoustical damping components (marginal to significant.
Arcicci Levitation rack, Billy Baggs rack significant.
Acoustical damping front wall between speaker   dense tapestry  Significant
acoustically damping components  DYI marginal but noticable
Closing eyes and darken the room significant 
then going deep into music nirvana  HUGE!
a good contact cleaner.
and probably (not yet fully tested)  tweeter diffraction (felt etc) , rings etc
AND HUGE IS PROPERLY WARMED UP GEAR!!! (both tube and SS!!)
and I am voting with Synergistic Research or this; Sun spots take you sound down a whole bunch of notches,
my experience, when I lived in Seattle, is that in damp cold fog  your system will sound dead and flat.