For clarity, you have heard both ST-10 and IDA-16(?), or IDA-8 and IDA-16? [still waking up/no coffee yet]
With luck and, perhaps, a little begging I hope to audition the ST-10 (or possibly dual STA-8s) in my system in another 5 weeks or so. I've been curious about the ST-10 for several months now.
I have not heard the IDA-8, but I have listened to the IDA-16 and ST-10, as well as the Bob Smith ST-10 SE mod. I'm listening to the overachieving pair of STA-9 & DAC-9 currently. Ranking the amps, in comparison to each other, the IDA-16 is on the cooler side, STA-9 warmer, and ST-10 in the middle. The differences are small but discernable, and since the NuPrime engineers can "dial in" the amount of warmth (add even harmonics ) as mentioned in this thread, you can choose your own desired temperature. I will say that if you don't find the Sabre chip DACs to be your cup of tea, the DAC-9 is a perfect foil, less incisive, yet still detailed, allowing less than perfectly recorded music very enjoyable, and a real bargain. Paired with the STA-9, they are doing a superb job in my system, great values can be found in High End audio.