Suggest a sub amp to power NHT 2.9 bass?

Two SA2 sub amps would work.

Also, THX rated amps have gain controls per channel. So you can use a normal 2 channel amp like the Parasound HCA1500 205x2 or the 1000A 130x2.

And yes, you would be passively biamping (and must remove the jumpers on the NHT 2.9 speaker terminals).

The mono NHT SA-3 was made not only to drive the SW3P slave sub, but also to be used in a biamped system the 3.3s and 2.9s or even drive two SW3Ps. It has adjustable gain, variably adjustable crossover and a phase switch. One thing though, unlike the SA-2 or MA-1 which accept both speaker level and line level inputs, the SA-3 only accepts line level inputs.

For many years I had used the SA-3/SW3P combo but in recent years found a combination of a Paradigm X30 and older Bryston 4B to be superior. But this combo would probably cost more than double that of a used NHT SA-3.

when you biamp with the SA3 or whatever method the 2.9's will open up considerably. I biamped my 3.3s with a EAD Powermaster 2000 (5 channels at 400watts into 8ohms- using 4 channels) and the active crossover in my EAD processor. It was amazing. Sealed woffers like current! Remember to remove the brass strap between the top and bottom binding posts.