I haven't heard the N803, only the S(N)800 / N801 / N802 / N805... The bass extension was solid to around 35 Hz but deep bass wasn't there. I compare the N802 with the SS30 and the benefits of the rest of the Sig 30 far outweighed the bass strength of the N802. In any event, I used a sub to supplement the bass.
In comparison to the SS25, there were differences between the two speakers. The response of the SS30 was smoothed out and considered better than the SS25. The quality of the drivers and crossover elements were of the same standards. The midrange and tweeter in both the SS30 and SS25 used silver voice coils, internal silver wire, and silver components in the crossover. The bass components of the SS30 did not use silver. The SS25 had an external crossover whereas the crossover of the SS30 was in a seperate box that was floated at the bottom of the SS30 cabinet.
Hope this helps...