" I like the 5150 resonance (1Meg pot and a .0068
     cap in series with the presence control). Can I
     simply insert the cap-pot network in the SLO
     feedback loop?" 

That's all it takes. For a while I had a resonance control in one of
my amps, I used a 250K pot and a .0047uF, if I recall correctly.
Eventually, I found I had no real use for it though. 

     "Is it the 22uF decoupling cap (in the 5150
     feedback loop) necessary for the resonance
     circuit?"

It is not necessary, but you may want to put it there since it blocks
the DC and prevents the control from being "scratchy."

     "About the SLO presence: this control is not very
     linear in my amp (from 0 to 5: no changes in
     tone). Is it possible to switching the cap? I'm
     searching for a darker or mellower tone and a
     more gradual pot."

No, the cap won't change a thing. It is the nature of the control.
You will see, the "problem" is that the Presence control works
"backwards," so to speak. If you grab a log taper por and wire the
presence control backwards, so that presence it's at maximum
when the pot is at zero, you will notice it works greadually, the way
you describe.

Some amps (Rivera-designed Fenders) used a 25K reverse log
taper pots, and with those the taper was nice and smooth. I have
never been able to find those pots though, and I have learned to
accept the fact that the presence control has a funky taper to it.

Cheers,

Gil 

"If I understand correctly: higher value of
     "resonance" pot = more resonance at max.
     setting?"

Right.

    " The resonance control has the same "linearity
     problem" of the presence control?"

No, becuase the resonance control is a series resistor (in the
feedback loop) that's either there (in parallel with a cap) or not
(shorting both the cap and the resistor), you can can wire it using
the GROUND lug in a log taper pot, and it has a great taper.

The problem with the presence control, on the other hand, is that
you can't use the ground lug (because we want the amp to sound
trebley when the control is open) using a log taper por is horrible (it
is anti log on the non ground lug), and a linear taper is not that great
either. That's why Rivera used the reverse log taper, and using the
non-ground lug, you end up with a log taper feel to the control
(which is what our ears like).

Gil