Will answer all your queries individually, but before that, a reassuring statement:
Your DAC and Amplifier will interface very well, even though the Volume control on your amplifier will need to be set pretty low, around the 9 O'clock position. You have Nothing to worry about.
A brief overview of your situation, first: The hand drawn block diagram below shows your DAC feeding your amplifier.
Every device has a input Impedance (think of it as a resistance, for simplicity) at its input and
an Output Impedance at its output.
Ideally, Input impedances need to be high & Output impedances need to be low so that all the Voltage being fed out gets delivered to the next input.
You have a Very good situation ("match") if the Output impedance of your source / sending equipment is atleast 10 Times LESS Than the Input Impedance of the next / receiving stage.
Based on the specs that you have shared,
1. The Output impedance of your DAC is a (nice and Low) 625 Ohms
2. The Input Impedance of the amplifier is 20K Ohms ( ie 20,000 Ohms , K=1000 as in 1 Kg = 1,000 Grams)
3. The DAC is sending out 2 Volts which gets divided between the 625 Ohms & 20,000 Ohms resistors acting as a simple voltage divider. The 20,000 ohm of the amplifier will receive 1.93 Volts out of the total of 2 Volts sent out by the DAC. Hence the Amp is receiving almost all the voltage sent out by the DAC, very little is wasted.
4. The amplifier needs ATLEAST 200 mV (milli Volts ie 0.2 Volts ) at its input, to provide full power.
As we have seen in point 3 above, the amp will receive (more than enough) 1.93 Volts.
5. The amplifier Damping Factor is 100 into an 8 Ohms speaker (specification mentioned by you), so its output impedance is 8/100 = 0.08 Ohms which I have shown in the block diagram above.
6. Now some Info on S/N (Signal To Noise) specification
i. The amplifier's S/N 103 dB means the Noise is 103 dB below (ie 141,254 Times smaller than !) the 40 Watt output! its not likely to bother you
ii. The DAC's S/N 115 dB means the Noise is 115 dB below (ie 562,341 Times smaller than !) the DAC output! It will certainly not bother you
iii. The DAC (115 dB ) S/N feeding the Amp (103 dB) will result in a combined S/N ratio of 103 dB... a very good figure under most circumstances.
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Now to respond to each of your queries individually:
a) The Amp requires 200 mv input signal at 20 Ohms to put out its rated power output of 40W pc to speakers?
Amp requires atleast 200 mV input signal at 20, THOUSAND Ohms, which is delivered easily, as explained in point 3 above
b) At 2V input voltage and at the rated output, distortion will become apparent beyond 103 dB?
103 dB is the Signal To Noise (S/N) ratio of your amp, Not Distortion. Distortion will be provided as a separate specification, most likely to be around 80 to 95 dB.
c) Relating to the dac output of 2 VRms, the amp can produce 40W output with SNR of 103 dB?
Yes, as explained in Point 6 iii above.
d) How does the 625 ohms output impedance from dac impact the input sensitivity for the amp?
Since 625 ohms is atleast 10 Times smaller than the input impedance of 20,000 ohms, its will have almost NO Impact on the sensitivity of the amp, as explained in point 3 above.
e) Does the high impedance of DAC output lead to lower voltage signal for the amp? Or to put it other way round- Would this then mean the dac's output strength to the amp would be lowered, thereby requiring more from power amp to reach reasonable listening levels (85db - 90db)?
As explained in your question d above, the DAC's 'strength' (signal sent) is not lowered significantly.
Even after the small loss from 2 Volts to 1.93 Volts, its still Much much larger than the 0.2 Volts required by the amplifier.
f) Would it be better to interconnect the dac and amps with higher impedance RCA 'cable' (100 Ohms instead of 75 Ohms RCA)?
You have a pretty ideal situation, you dont need to change anything.
... and Incidentally, Analog RCA Cables do not have any well defined / specified impedance, but that is another story all together.
75 Ohms is the specified impedance for Digital Cables carrying Digital signals.
g) How does the SNR of both DAC and AMP interact here - i.e. good or bad?
If 2 pieces of Cascaded equipment have S/N more than 6 dB apart ( in your case 103 dB & 115 dB, then the Lower S/N ratio is thge effective combined S/N ratio. I have mentioned this in Point 6 iii above.
103 dB S/N is Very good for your application.
I will need to feed it a signal of atleast 70/4 = 17.5 Volts with the Volts with the Volume on Full!
