> i don't currently use the 5v for any of my modules. if my memory
> serves correctly, there is one rail used by the full-sized 16-pin
> (opposed to the 10 of 8 pin) connector that carries +5vDC as needed,

yes

Right. However....if you mount 3 of these modules in a Doepfer
rack, that will add up to perhaps 1800 mA of load. That might
be better delivered by separate wiring, although I think the
existing 16-pin power busses should be able to handle a couple
tube modules.

NOW, do you see why there are no tube Euro modules

It's POSSIBLE to use +-12v only. It requires the usage of
tubes with 12v or other "odd" heater voltages, which are
easy to get but not as commonplace as 6-volt heater versions.

(I'd like to use 6GH8s if at ALL possible. At one time,
the 6GH8 was the one tube in the world manufactured in
the largest quantities---
mainly because it was used in TV sets that ran them
very hard, and made for a short lifetime.
Lud Sibley of the Tube Collector's Association owns the
RCA corporate archive. He did the research in the files
and found that in 1960-61, RCA was making FOUR MILLION
6GH8s PER MONTH. By comparison, they made "only" about
100,000 12AX7s per month. So, leftover 6GH8s are
everywhere today.)

And the total power consumption will be the same anyway.
A typical two-tube module will use about 3-4 watts just
for tube heaters, regardless of where the power comes from.
So it's your choice:

1) get the cab modified to hold an extra +5v supply,
or
2) use large +-12 power supplies to run the tube load.

And there's another headache:
Want a Frac version of these modules Well, Frac systems
usually have +-15v. No problem for the solid-state support
circuits. But the tube heaters will HAVE to have optional
dropping resistors to keep their voltages down.

More complexity, more PC board space.