\rHi Mike:
The Boom Tomb was the name of a Speakercraft In-Ground Subwoofer that they recommended a 250 Watt amplifier. Niles has in an in-ground 10" subwoofer rated at 50Watts. At one time these companies were fierce competitors, but both were purchased by the same company back in 2007 along with 15 other companies including Elan, Proficient, Furman, Panamax, and Sunfire. Today both Niles and Speakercraft are part of the Core group a Nortek Company. I did find a 12" Niles in-Ground sub rated at 8-ohms and 500 Watts. Its Sensitivity Rating is 86 dB which tells us you need more power to reach full output. I would suggest that the SMP500 would be a better choice for the Niles in-ground sub.
1) The SMP500 will do 500 Watts peak at 8 ohms
2) It is Mono output already bridged.
3)12 gauge for your low level is ideal. I do not know enough about your configuration to comment. You do not mention anything about possible speaker selector, Volume Controls, Impedance Matching or the model number of the ONKYO and how you are connected.
tim@osdaudio.com
TL/5510\r