The two subwoofers are very different designs and price reflects their relative merits and performance. There really isn't much in common between the 2 subs except for the size of the driver. The 500 has the superior driver with its larger magnet and 3" dual voicecoil. The dual voicecoil allows for lower impedance and higher current from the digital amp.
The DSP controlled 500W proprietary amp in the 500 is arguably its finest feature. The 350's smaller 200W amp will strain in comparison particularly at high SPLs down low. The 500 amp mates very well with the 12" driver in the larger enclosure which results in low THD. Its very flat FR linearity is matched by few subwoofers anywhere while the 350 while flat to 30hz runs out of gas at levels 10hz higher than the 500.
The 500 also has XLR connections which are useful for eliminating noise over long runs if connecting to a receiver/processor with balanced outputs. Also the large single flared vortex port provides much more bass output with less port noise than the dual circular ports in the 350. Listening tests usually rank the 500 very highly in terms of articualtion and clarity when compared to other well-known subs in its price range. The 500 really is an outstanding subwoofer design by any standard. The 350 is a good sub at its price point but in this case its not diminishing returns when stepping up to the 500.
