Here’s the formula for determining the value of the capacitor (in pf) you need to solder onto your amp’s input jacks (the interior of the amp is the best location) to achieve the 6dB/octave filtering:
1/(2 x pi x f x 2), where f = the desired "corner" frequency of the filter.
100Hz maximum is typical, but you can go lower if you want less sub, higher if you want more power from your Quicksilver available for the ET’s. Many subs are not recommended above 100Hz (the mass of the woofer cone being too high), the Rythmik F12G one notable exception. Also, go above 100Hz only if you have stereo subs, as localization of the sub(s) will become audible above that, perhaps even AT 100Hz. The lower in distortion a sub, the less will it be audible as a separate sound source.
An active electronic sub with a steeper high pass filter (to the power amp) will make more power available for the ET’s, at the possible cost of a slight loss of transparency, depending on the quality of the x/o.
If you’re VERY ambitious, you could get a pair of the remarkable GR Research/Rythmik OB/Dipole Subs and use them in place of the stock ET 8" woofer, crossing-over at the factory 180Hz (the sub can be used up to an incredible 300Hz!). State-Of-The-Art bass reproduction!
