Ask a hundred session players, get a hundred answers. The difficulty with this sort of conversation is that everyone has their own idea of what good tone is.
To me, good tone is what fits the particular track I'm working on at the time. That's why I usually sit in the control room whenever possible. You lose the pickup/amp interaction, but the perspective you gain is well worth it.
hmmm...what else....
a 1x12 Thiele (design) cabinet with an EVM-12L is a pretty safe bet in the studio.
slaving amps to an external poweramp is neat trick. Why do it? A cranked 50w or 100w amp can over-power a room pretty easily. Putting a loadbox (like the Palmer PGA-04) in the equation allows you to crank the amp, while maintaining a reasonable volume.
"Nashville tuning" comes in handy from time to time. It's standard tuning, but using the octave strings from a 12 string set (of strings). I usually make my own set from single strings; I find even the heaviest set of 12 strings are way too light for proper intonation.
Another useful tuning is DADGAD (using standard strings). Of course drop-D is great too.
One handy gadget I tried yesterday was a "jellyfish" pick. Hard to explain what it does, except to say it's great for bright, shimmery parts.
Sometimes rolling the volume back on your guitar makes things punchier.
Robert Keeley makes a great sounding compressor (pedal).
Teles seem to twang better with .009 gauge strings.
That's all for now.