When you open the NFB loop, you are increasing the
over-all circuit gain by a factor of five in a Marshall, or
by 1.5 in a Fender. This means that ALL signals coming
into the power amp will be amplified by this extra
amount-- beautiful guitar notes and ghostly noise, too.
 
One way to decrease this noise is to reduce the gain
somwhere else. Try lifting one end of a cathode bypass
cap in the preamp. This will reduce the gain of that
stage by at least two. Depending on the cap value-- if
large, then no real worries-- you might alter the voicing
of the preamp in the process.
 
You could also add an attenuator between the preamp
and power amp. This could give a straight signal loss
equal to your open-looped PA's gain "increase". You
might then add some caps to the attenuator to sweeten
the tone, as some brightness may be lost.
 
Another noise reducer, is a regulated supply, but gain is
your main concern.
 
Have fun
Kevin O'Connor

Lots of people look at NFB as a way to increase the
over gain of an amp. Another way to look at it is that it
can also be a "balancing" act. 
 
If you remove the NFB loop on a Marshall you loose
that marshall quality that people like so much. It's the
midrange "punch" so to speak. Once the feedback loop
is removed the bass and treble are also increased so the
midrange frequencies tend to go down, or so it seems.
You notice a big difference when you palm mute on a
Marshall.
 
Fender's use the 820 ohm value in most amps and that
also lends itself to the Fender "Spank" as far as clean
tones go. Feedback is a VERY deep topic and novels
could be written on it. 
 
Trace

Which is why I think the best way is to put an 820 ohm
resistor and 100k trimmer in series in the NFB. Tweak
to taste.
 
Getting on to that hiss, though, I cleared up a similar
hum in a guy's amp this weekend. It was always pretty
noticeable, and it stayed at the same level no matter
what the volume settings were. I cured it by replacing
the leads to the master volume pot (pre-pi design) with
shielded wire.
 
Shea

I still like putting a 50K pot (wired as a rheostat) in place of the larger resistor in the divider and mounting it in the ext spkr jack hole. It's a fun adjustment to have, and is easy to reverse.

... and to think you only scratched the surface of
NFB... I do tend to prefer more NFB and not,
however... i really don't see why so many people like
no NFB on an amp... I mean, take a 5e3 circuit... not
bad at all, now add a bit of NFB, even if filtered thru
some series parallel resistors/caps to allow for a shapng
of the frequencies fed back, and it really seems so
much more controlled to me... THe pot/resistor idea is
something I use a LOT when coming up with a new
design, or even tweaking an amp with aq different OT
etc.. really lts you dial in a sweet spot. Glad to see you
found what you liked, however