Best wire to replace stock wiring in preamp?

Overhang, I believe strongly in solid-conductor cable, not stranded. Yours is stranded. I'd use the same thing I recommended just above.
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Solid core dead soft silver is what to use, minimum four nines purity. Don't be tempted to try jeweler's silver. A couple of sources are listed above, although I can't say what the differences are between them.

Leave the wire alone. The problem from brighteness is not
your pre-amp, its from your digital source. The frequency
measurments from your P/LD 2000 are so flat it will reveal
all flaws from your D/A converter chips. Companies like
Wolfson from England, and Cirrus Logic of Crystal Semiconductor in Texas, are producing the warmest, richest,
most analog sounding DAC's on the market. Marantz, Arcam,
Integra, Integra Research are using their DAC's. Stay away
from brands using Burr-Brown chips. Their DAC's are as bright as a screaming witch. The best bang for the buck is
a killer pro model from Marantz, the PMD 325. It uses the
Cirrus Logic D/A chips. This player has single ended and
XLR outs and also has pitch control. It is very warm and
smooth. Go to www.fullcompass.com in Wisconsin and you can
pick one up at a good discount. If your on a budget, and want a warm player, this is it!!!

And just why do you think that B-B DACs "are as bright as a screaming witch"? Perhaps you would care to share this with us, as well as your design experience and insight.

Reason being........I don't buy what you are selling.

The transient speed from Burr-Brown dac's is so
fast, that it throws the music so far out from
the speaker, that its like being slapped in the
face, and this can be a real problem in small
living quarters. I still have my old 38 lb Denon
player, which has four Burr-Brown dacs, but it
is no match against my Marantz PMD 325 player
which uses a Cirrus Logic 4396 dac. I always
test players with my older recordings, Mozart
pieces on the Deutsch Grammophon label of the
Berlin Philharmonic Symphony from the late 60's
thru the early seventies. Mozart loved to compose
in a high pitch. An audiophile friend brought
over last winter a Denon pro model which has Burr-
Brown dacs, and put it up against the Marantz.
Thru the Denon the upper frequencies were bright,
shrill and on the cold side. Thru the Marantz,
the highs were much sweeter and smoother. Crystal
Semiconducter and Wolfson take a different approach,
they slow down the transient time focusing on
more body and weight to each instrument, producing
a fuller, smoother, more analog sound. This is wht I
used the term screaming witch, and that daltonlanny
should consider using a player with slower transient
time, which will solve the problem of a bright preamp.