Reel to reel deck directly to power amp ?

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?

Yes. However:

Generally speaking a problem is likeliest to arise in cases in which the amp is high powered (and most or all of its power capability is actually required by the user, at least occasionally), since there tends to be a **loose** correlation between the power capability of various amplifiers and the input voltages required to drive them to full power.

Usually higher powered amps have more gain. This is because they are often used on lower efficiency loudspeakers, and the gain is need both to make use of the preamp signal and also to make the voltage swing needed to drive the output devices. The result is that with a pro audio tape machine usually has more than enough drive to work in most situations.

Depending on the machine, if its output is not loaded at 600 ohms, it may well make more voltage. At any rate I've run tape machines directly into power amps many times with no worries. The exception as you correctly point out is that older consumer decks often do have lower outputs, the standard for many cassette machines was 1 volt. But I think we aren't talking about consumer machines here.

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.

:)   We offer tape EQ in our MP-1 and MP-3 preamps and they work pretty well with our amps...

Ralph, that's good to know. Do you offer both IEC and NAB equalization ? Do I need both ? At least for pre-recorded tapes, I guess. In what way do they differ ?
The deck must be modified to use outboard head preamp, I think it can be modified to include a switch for use with both internal and external playback head preamp.
But..with Ralph's preamp I would have to use tape-outs to record from vinyl, something that I like to avoid, unless of course Ralph will guarantee that there will be virtually no signal loss. 
Ralph, do you think your 30 watt/ch amps would drive something like Harbeth or Dynaudio, thinking medium size room and not thinking insane dbs ? 60 tube watts should be no problem, not forgetting speakers' impedance curve.

@inna
Our 30 watt amps don't do very well if the impedance is less than 8 ohms.

IEC and NAB are a good idea. At 15 i.p.s., the difference is basically where the turnover is; NAB is at 3150Hz while IEC is at 4500Hz. There has been debate over the decades about whether or not bass emphasis is needed for the NAB curve and some pretty good arguments for why its not seem to win out. So the playback curve is essentially a 6db per octave rolloff; the designer has to choose where that should start. We have enough gain that we can do it at 16Hz.
To use your machine for record, you would need a switch on the playback head which directs the head signal to an XLR or uses the internal electronics. To use our phono circuit for recording onto this deck, I recommend a Jensen transformer to make the match- the phono section otherwise by itself will be bass-shy driving a 10K load.

One more cable plus transformer - this will degrade the sound. Separate phono with appropriate output impedance will be better. What is the good range of output/input impedance proportion, 1/5-1/10 or so ?
I still don't understand IEC /NAB real difference and whether or not I may need both. Both Studer and Otari electronics do have both.
Anyway, there is so much that I don't know or even have no idea of when it comes to open reel decks.

I have an "old" Crown SS800 reel-to-reel that I use in one of my systems.  It has all of the inputs and outputs associated with a pre-amp and is what I run the amps with.  The difference with your Studer / Otari concept is it/they don't have any tone controls (yeah, I know, audio purists don't "do" tone controls.  I do.  Even have a graphic equalizer in my esoteric system - gasp!), which may make your listening a little strained.  The Crown also has Class-A inputs (courtesy of John Haynes, Goshen, IN - Wherever you are dear friend) that sound really great with older/vintage speakers.  I'm partial to co-axial/coincidental speakers also.  So good luck with the system and I look forward to hearing about what your hearing....brilliant!