Preamps... understanding volume...

they amplify, and they attenuate. 
using simple round #'s, a bit of history. Legacy equipment/terms are in use but ....

1. original mono sources (radio tuners, ceramic phono cartridges)  provided strong enough signals directly to amplifiers, let's say 1 volt. This strength is called 'Line Level'. (newer, like CD's provide more, i.e. 2 volt or more). Pre-amps did not exist.

All sources producing 'line level' can go direct to an amp without a pre-amp, the only consideration being volume control (attenuation).

2. early stereo, tapes in 1956, your system being mono to begin with, simply add another mono amp and speaker, both amps receiving 1 volt. LP went stereo in 1958.

3. enter low voltage stereo signals. a. tape head (low signal direct from the tape/tape head). b. phono magnetic cartridge signals (phono low). pre-amps were ADDED, to pre-amplify the new low signals up to 1 volt, then into the original amps. Thus they amplify.

4. both new low level signals, tape and phono ALSO needed/still need 'equalization'. Standardized recording/playback systems were developed, NAB for tapes, RIAA for phono.

Phono as example: (tape recording/playback a similar process, a different 'eq' curve).)
Phono initial physical recording cuts lows (minimizes width of groove needed for bass), AND boost's highs (gets signal further away from noise, like later Dolby does).
LP. Long playing. Cutting the bass grooves allowed more grooves, thus 'long playing'.

Phono Playback needs an equalization circuit, to boost the lows and to cut the highs, before it goes to the amplification portion of the pre-amp, to get up to 1 volt to go to the amp. 

Phono 'moving coil' came later, weaker signal, needing a pre-pre amp so to speak, to get it up to phono low, then to pre-qamp's eq and boost uo to line level for the amp.

'Tape Head', signal directly from the tape, is low level, needing both NAB eq and pre-amplification, thus separate inputs. 

'Tape' is line level, already equalized and strength boosted, by the tape recorder/player's electronics, therefore it can go in any tape input, tape loop, or straight to an amp (again, how control volume?)

5. Attenuation (volume control): various sources produce a variety of signal strength, they need to be 'adjusted' prior to going to the amp, thus a pre-amp volume control. Implementation varies, cut/boost/bypass, ....

McIntosh and others provide 'trim' controls, which individually 'pre-adjust' those low voltages 'to match each other's signal strength' minimizing the need to adjust the pre-amp volume (switch inputs-get same volume).

6. Headphones: some pre-amps have separate headphone amplifiers. Never goes to the amp.

7. Rare Pre-amps, like McIntosh C26, use the headphone amplifier circuit, say 10 wpc, to directly drive efficient speakers. Like future receivers, they offer 'main, 'remote', or both simultaneously.

http://www.berners.ch/McIntosh/Downloads/C26_own.pdf

8. Add a tuner to the pre-amp, like my new to me mx110z

http://www.berners.ch/McIntosh/Downloads/MX110_own.pdf

9. Add multiple inputs, switching, volume control to the amp, get 'Integrated Amp, like my new to me Cayin A88T. (I needed MK1 to get 16 ohm taps from the transformer, rare these days).

https://www.elusivedisc.com/CAYIAA88T.pdf

10. Add a Remote Line Controller, inputs, volume, balance (I love squeaking the balance speck by speck from listening position), tone, .

https://www.ebay.com/p/Vtg-Chase-Tech-Rlc-1-Remote-Line-Controller-Upgrade-Stereo-to-Wireless-IOB-J588/1424512686

What a complicated web I have built for myself, selling my Digital pre-amp and amp and getting tube pre-amp, tube tuner, tube amp, all with various methods of control, direct and remote.

Oh yeah, you young folks need to deal with modern digital sources, and modern headphones.

forgot this link to phono eq

https://en.wikipedia.org/wiki/RIAA_equalization

To the OP: I took a look at the specs and descriptions of the six source components you listed. Some comments regarding driving the Odyssey Stratos Extreme directly from them:

1) Note that the RME device only provides balanced outputs, and see the comment regarding the Stratos that was provided by Stereo5 on 6-4-2018, and my comment immediately following his post, in this thread.

2) The lowest of the three output level settings provided by the RME (+2 dbV balanced) may be too low to be suitable, depending on the gain and sensitivity of the amp. At least one of the other two settings should be ok, though.

3) NAD apparently does not provide a spec for output level or for output impedance when one of the digital inputs of the C658 is being used. I suspect that it would be ok in both respects, though, unless the amp’s gain and sensitivity are considerably different than average (see note 5 re "average").

4) The miniDSP device is the only one of the six for which an output impedance spec is provided, and it would have no problems driving 22K. I suspect the others would also be ok in terms of impedance compatibility, but to be certain you may want to check with their manufacturers.

5) The output levels of the Topping, miniDSP, and two ProJect models will all be fine unless the gain and sensitivity of the amp are either well below or well above average. Average gain for a power amp tends to be in the vicinity of 26 to 30 db or so; for an amp like this which is rated to provide 150 watts into 8 ohms those gains correspond to sensitivities of about 1.7 and 1.1 volts respectively.

Good luck. Regards,
-- Al

volume control of tape line level signal directly to amp:

It is not good to use the tape recorder's volume control, because, you want the strongest un-distorted signal off the tape (furthest from the noise), The meters help you do this, you can go into the red for short bursts. You get the strongest un-distorted signal from each tape to start.

My office Carver cube has no volume control, tape level too loud, so I needed to add intermediate volume control, thus Chase RLC-1.

Thanks to everyone for all the info... far more than I expected!