There's nothing to prevent you from using the NS-F51 speakers with the AV receiver, but the tonal signature isn't likely to match that associated with the remaining HK speakers. The Yamaha speaker will more than likely perform better than the HK sat speakers though and should marginally improve stereo performance.
Thank you for responding and apologies for not being clear.
HTR 2071 have a RMS power (20Hz - 20 kHz) 70W 6 ohm 0.9 THD
NS-F51 Nominal power input 80W
I'm using the setup for watching movies and sometime listening to music as a background to other activity. The HKTS-11 perform reasonably well for movie, but not so for some of my song collection. And looking for NS-F51 to make it better.
So my question are actually
1. does the spec of HTR 2071 match with NS-F51?
2. Will changing the front speaker degrade the movie watching experience? (As you said the NS-F51 will improve the stereo performance)
The specifications don't need to match. The AV receiver would be able to power the Yamaha spears without the speakers having the same specifications as the HK speakers.
No, there's no real reason why the sound or audio would be degraded. It should in theory sound better. The only issue may be the tonal signature not matching that of the HK centre speaker. This may be detectable as audio pans across the front soundstage? It is ordinarilly suggested that you try keep the centre and the left and right speakers the same brand and from the same range in order to try maintain tonal matching.