Preamps... understanding volume...

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!

the relationship of strong or stronger signal going from preamp into an amp, and whether the amp can therefore produce more power for low sensitivity speakers (or any) has not been answered.

I do not know for sure, but

Isuspect the amp cannot produce more than it's maximum GUARRANTEED wpc (they often produce more power in spec than their official rating).

McIntosh preamps I have owned output 2.5v (all?). McIntosh ampsI have owned have two optional inputs (all?): alternate .75v and 2.5v to match their preamp's 2.5v. The amp only has one set of published output rating for each ohm tap (4, 8, or older equipment 16 ohms). 

They do not have lower or higher ratings depending on which input level is used. 

I always advise, whenever you buy, or switch equipment, try to find more efficient speakers you love. High power amps do not/will not damage speakers, low wattage amps can damage speakers. Efficient speakers set you up to only need/try lower power amps, especially tube amps.

Amp GUARRANTEED WPC. I owned an mc2250, rated for 250 wpc. Testing at lab day at Harveys proved it produced 320 wpc within spec. The McIntosh technician said "sir, you have an unusual unit". Printed out spec sheet showing the results, changed the McIntosh decal on the back, signed and dated it.

You never know, certainly cannot plan on it. McIntosh and NAD amps both have protection circuits to protect your speakers and themselves. They get engaged when asked (by the speakers) for more than their rating. Others probably do too, I just have no experience with others. Indicates you need more power, or more efficient speakers.

Amps are limited, in absolute terms, by

• The voltage rails in the power supplies. These provide the maximum + and - V swing.
  
• The current available from their own power supplies, before those rails sag
  
• Thermally limited in the amount of power they can output before they shut off or melt.This is why peak power may be much  greater than continuous, as well as limit the ability to deliver into low impedance loads.
  

Amps are also a fixed voltage gain. Often around 28 dB. So:
Output = ( Input + 28 dB ) OR ( MAX)
whichever is less.