Amplifier design parameters required for a good match for high (16+) ohm speakers?

Ralph, I am using the 16ohm output with the Contrasts, which are rated at 16ohms. I am not sure about actual impedance within the bass frequencies.

Which model?

Ralph, I haven't put many hours on my ST-70 since I had you refurbish it in spring of 2016. However I now fell in position to do so. Took a long time for me to find a complimentary set of  speakers I could afford.  I have never any doubt that my ST-70 would prove reliable. As an aside, how do the St-70s fair when used in mono mode? 

Speakers are Contrast Model One AS3 Ref. 

The Model One should be very nice with the ST-70. They rate the sensitivity at 92 dB if memory serves; if that is actually sensitivity, the speaker is 95dB 1 watt/1 meter, which will mean the ST-70 has adequate power. If the amp has not been used much, it will need some break-in time during which is will become more lucid.

By mono mode, do you mean monostrapped?

Ralph,
I find the paring of the Contrasts with the ST-70 to be quite adequate in my 12'x15' acoustically treated room. Bass has improved with playing time. The Contrast are certainly revealing such that for my tastes best mated to warmish SS or tube amplification. It was their 16 ohm impedance that drew me toward the purchase. 

Regarding the mono mode question: I am asking about your experience using two ST-70s as mono amps. I believe this is strapping.  Based on the owners manual this can be done.  

Forgot to add that the manufactures rated sensitivity was based on 1 watt, not 2.83V.  This is what seems consistent with the loudness I obtain when comparing to other 8 ohm speakers having 88-89dB sensitivity at 2.83V.  My assessment is based on comparison with equal volume setting on different preamps, integrated amps. Not a true measurement. Again, I was more concerned about the impedance factor than the sensitivity as I have a small listening room and listen in a near-field environment.