I have read some posts about the V72 being a dark preamp, but it seems
to me that any V72 which sounds dark simply has bad power supply
electrolytics, if the tubes are new and the broadcast filter variable
cap (15khz) has not deteriorated or been tampered with.

In a class A design such as the V72 the power supply capacitors are
part of the audio circuit. This is primarily the case for the 8 uF cap
that feeds the inductor for the output stage.
The V72 has a fair amount of neg. feedback that will compensate for
the aged components and tubes, and will correct the deficiencies of
the amp more or less depending on the signal that it amplifies, but
there are inherent distortions that will never be eliminated without
replacing the 8 uF caps for the P.S.

Aged electrolytics will lose their ESR and act like an inductor on
high frequencies. This means that on sharp transients and high
frequencies the filter cap becomes 'open' and will not supply current
to the circuit.
All electrolytics age with time, and NOS electrolytics will age in
some instances faster than regularly used electrolytics.
Any capacitor manufacturer will provide a usable shelf life at a
certain temperature and this life is less than a decade.

40 year old electrolytics will not sound the same as they were 10, 20
or 30 years ago, just like old tubes.

Many internet stores sell V72 amps which are racked for high prices
but with old tubes and caps, and then the buyer may find him or
herself asking why the preamps sound dull and distorted.

The amps themselves are very clean sounding once they have been
restored to spec.