Was there any changes in the presence 
     circuit/negative feedback loop between the 
     68 plexis and the 70's models? My 70's
     plexi
     is so icepicky... not smooth at all... 

The point to point 73's usually came with .68uF
presence cap which I actually prefer over the common
.1uF, which is pretty much stock in all Marshalls. 

Aside from that there wasn't any real changes. Some of
the Metal faced amps that were imported in the US
came with 6550's and the feedback tap was on the
4-ohm tap. Some guys like the feedback on the 4-om
tap. It's pretty much personal preferance really.

As for the Ice-Pick thang...You can try this setting.

Bass: off
Mids: 6
Treble: off
Presence: off

Both channels at 5 and plug into the treble channel's
high input jack and jump from the treble channel's low
input jack to the Bass channel's high input jack. (whew,
there's a run on sentance!! ha, ha)

That's a setting that I've found to work great for stock
4-inputers. If you want to mod it there's a mess of
things you can do. 

1.) Remove or lower the bright cap on the treble
channel's volume pot. 

2.) Remove or lower the 470pF mica cap that is in
parallel (piggy backed) on the treble channel's 470K
mix resistor (see OEI's site for a photo). I like to
replace the mica cap (usually red) with a ceramic cap of
the same or lesser value, depending on the plate voltage.

3.) Higher plate voltages will make the amp brighter and
give you more headroom where lower voltages will
make the amp warmer or "browner" as well breaking
up earlier. 

4.) Sometimes the Treble channel's coulping cap is a
.002. You can change this to a .022 which also helps. 

These are a few of the most common tricks to try.
There's others but these usually work pretty well. Try
one and see if that cures it or try them all! (ha, ha)
That's the beauty of tone, it's all subjective. I'm sure
others will pop in with their favorite tricks, I hope this
helps!

Trace