You could just biamp using the 4 channels of the 7100 but I think the advantages of a sub amp (whether nht's or some ther brand)would be the gain adjustment, phase control, high and low pass crossovers.

Your 2.9's have passive crossovers which are not bypassed when you biamplify. The crossover is: 100 Hz 12dB/octave high-pass and low-pass, 320 Hz 12dB/oct. high-pass & low-pass
and 3.5 kHz 18dB/octave high-pass. Make sure you remove the connection straps on the binding posts before you biamp.

All that being said it isn't imperative that you biamplify I just figured if you didn't have a seperate sub you might want to have a little more control over the sub sections of the 2.9's for different listening stituations such as music or the lastest blockbuster.