Reel to reel deck directly to power amp ?

@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!

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 ?

Ideally 1:10 or better is the rule of thumb.
It will be tricky to come up with a tube phono section that is OK driving 10K ohms. While the output impedance might look low enough on paper, the defining spec is what the output impedance is at 20Hz, not 1KHz.

That is why I recommended the transformers. BTW, input and output transformers are quite common in studio gear, from whence most of our recordings come. I don't like transformers, but they have their place and when they are needed, they can be really helpful!
Where will the extra cable be used??

Ralph, unless I got confused - one cable goes from preamp's tape-out to transformer and another from transformer to deck, as opposed to from separate phono directly to deck.
I think, my Nak's input impedance is around 50 kohm and my Acoustech's output impedance is close to 10 kohm, very high. They match very well, I can tell this.
Hard to find phono stage to impedance match pro or semi-pro deck...
Unexpected complication.