Unfortunately it is expensive to produce ultra low frequencies with minimal distortion, an order of magnitude more costly than for bass/mid frequencies and two orders of magnitude harder than for a simple tweeter.
This is why a good sub usually costs so much.
If you wish to purchase an active sub woofer with similar distortion figures as attained in your passive high-end speakers then you should expect to pay at least as much as you paid for the pair of high-end speakers. The sub typically needs three or four times more amplification than a single speaker and includes a heavily damped cabinet, large driver and adjustable x-over settings.
As far as I know, there is no cheap way to get low distortion sound in a sub. If this seems like a crazy expense just to augment a couple of the lowest octaves in the audible range then it is quite sensible to remain without a sub. However, I wouldn't recommend the cheap sub coupled with expensive high-end speakers as the sub distortion will indeed mask one of the major benefits of a high-end speaker, which is a good clean and accurate bass.
This is why a good sub usually costs so much.
If you wish to purchase an active sub woofer with similar distortion figures as attained in your passive high-end speakers then you should expect to pay at least as much as you paid for the pair of high-end speakers. The sub typically needs three or four times more amplification than a single speaker and includes a heavily damped cabinet, large driver and adjustable x-over settings.
As far as I know, there is no cheap way to get low distortion sound in a sub. If this seems like a crazy expense just to augment a couple of the lowest octaves in the audible range then it is quite sensible to remain without a sub. However, I wouldn't recommend the cheap sub coupled with expensive high-end speakers as the sub distortion will indeed mask one of the major benefits of a high-end speaker, which is a good clean and accurate bass.