The "Low-Frequency Crossover" control on the back of some powered subs, like my Velodyne CHT-8 (Inexpensive by the way at only about $350 list) is actually a control knob labeled with frequencies from 40-120 Hz. It actually governs the operating range of the sub. Mine is "dialed in" or set at about 120 Hz (high on the scale) because I have small satellite speakers. If your sub has this control, you can vary its setting depending on which speakers you use with it. But, it's not really a true crossover in itself.
With no other crossover, you are actually running your sats then at "full range", with the natural roll-off of your sats sort of acting as a crossover. The problem with this arrangement, that is, with no crossover in your receiver and none in your sub (perhaps) is that you are feeding full-range signals, including low frequencies to your satellites. Even though your sats may naturally roll-off the lows at, say, 90 Hz, they may still be straining to produce bass.
The control mentioned above does not solve that problem but merely sets the subs operating range. So if you set it at, say, 100 Hz, and your sats roll-off at 90 Hz, you kind of have a smooth transition from sats to sub. But again, your sats are still being fed full-range signals.
With a true crossover, also called a "high-pass" crossover, you are only sending certain frequencies to your sats. Let's say you have one (in the sub or your receiver) and set it to 100 Hz. That means that the sats are receiving input at only 100Hz and above, sparing them from producing low-frequencies better handled by the sub.
You may have such a control in your passive sub, but it may be non-adjustable. When you use speaker-level hookups (ruuning speaker wire from your receiver to your sub, then to the sats) the sub likely uses a fixed internal crossover, and since your system is a Cambridge and was packaged together, the manufacturer may have chosen a crossover that fit best (maybe 120 Hz in the case of very small sats).
Running a seperate amp to your sub won't change any of this, unless your amp has its own crossover. Even if you buy a home-theater amp with 5 channels, it probably won't have a crossover because that's a function of a pre-amp (or the pre-amp....ie "control" section of a receiver). So I doubt that using an amp to drive your passive sub will change anything.
Keep in mind too that many new home-theather receivers have crossovers that may not be labeled such, but rather are enabled when choosing your speaker setting. For example, choosing "small" (as in small sats) engages the crossover in the receiver, which may be fixed at, say, 80 Hz. Or, some receivers do actually allow you to adjust the crossover frequency. For a pair of Paradigm Titans, for example, you probably could dial-in (adjust) your crossover to 60 Hz, since the Titans have a 6.5 in woofer and can handle bass down to 50 to 60 Hz or so with no strain.
