Reel to reel deck directly to power amp ?

Yes, I was thinking about output/input match, but Ralph didn't mention there might be a problem.

With any pro audio or semi-pro reel to reel, you have a volume control to control output. On my Otari 8-channel, not so easily accessed as you have to put a screwdriver though a hole in a panel. But the two channel machines have a regular volume control and are typically set to at least +4 dbm at full volume (some might be at +10); either way that's more than plenty to clip any amplifier made, and the output is robust enough to drive any amplifier made.
IOW just do it :)
I of course prefer tubes, and used to do this with my Ampex 351 machine. It was a bit harder, as the volume controls were on each channel and so a bit harder to adjust. But more modern machines have made that bit easy.

An output to control volume to "pre-amp", not power amp..

Adjusting volume with a big bloody screwdriver would be cool !
I prefer tubes as well, we'll see. 
Next step could be asking Ralph to custom make playback head tube preamp or maybe playback head/phono stage unit, and match it precisely to his amps, and match the amps to speakers. Now that would be high end.

I took a look at the manual for the Studer A810 that was mentioned in the OP. (The manual can be found at hifiengine.com if one is registered at the site).

Depending on the version of the deck and on how the magnetic flux reference level is defined the maximum output is either +10 dbm, +16 dbm, or +24 dbm. Apparently only balanced outputs (and balanced inputs) are provided.

Ralph, would I be correct in thinking that these dbm values, as well as the ones you referred to, mean decibels above a milliwatt into 600 ohms?

If so, +10 dbm corresponds to about 2.45 volts; +16 dbm corresponds to about 4.9 volts; and +24 dbm corresponds to about 12.25 volts. The lowest of those numbers won’t be enough to drive the balanced inputs of **some** power amps to full power, although it would be enough for many and probably most of them.

Output impedance, btw, won’t be any problem at all in this case, as the deck is specified as having an output impedance of 50 ohms max across the frequency range, and as being capable of driving a load impedance as low as 200 ohms. However the line-level input impedance is only 10K, which would not be optimal for some sources, especially some tube-based ones.

Orpheus10 11-13-2018

The RS 1500 puts out a max signal of .775 volts, while a good pre puts out a max of 20 volts for the power amp.

To be sure it’s clear, preamp output specs that are stated to be in the tens of volts, or even in the high single digits, represent the preamp’s maximum output voltage **capability.** A preamp’s actual output voltage will be the input voltage it is provided with multiplied by its gain (expressed as a ratio, rather than in db), and reduced by the amount of attenuation the volume control is set to provide. No preamp in any home audio system would ever be called upon to provide an output of more than a few volts balanced, and usually not more than one or two volts unbalanced.

From my point of view, this is a case of the blind leading the blind, and then running with it.

From my point of view, this comment is uncalled for as well as inaccurate. I commend Ralph for not responding in kind.

Regards,
-- Al

Otari MX5050 BII2 manual says +4db max output. This won't be enough in many or most cases, correct ?
One more potential issue, if phono stage has only RCA outputs I would lose 6db of gain when using RCA to XLR cables or adapters.