The question is pretty broad, so I'll try and cover all the various answers.
(1) You can find prosound active crossovers that are 2, 3, or 4 way mono and stereo units that allow for easy adjustment of the crossover point and levels. These use state-variable circuit designs, and are typically considered to be inferior sonically. They also are designed for pro-level signal levels (+10v instead of -4v, though the scales of each don't exactly correspond) and
may have problems integrating with consumer gear. In my experience, I've found that the input and output gain structure of these units allows for them to be completely compatible with consumer gear, even if the solution isn't "ideal." I've also found the impact on sound quality to be negligible... probably because any noise or distortion introduced by the state-variable circuit is swamped by the benefits of using active amplification and removing the frequency dependant nature of passive crossovers. Behringer, Rane, dBX and others make these crossovers... I have used the dBX in the past. I should note that I'm talking about analog crossovers here. These companies also have (most do, anyway) digital crossovers that include an extra analog/digital and digital/analog stage. These offer a tremendous increase in flexibility (user adjustable/programmable filter types, points, orders, levels, digital time delay, etc.), but may sacrifice sound quality due to the A/D and D/A stages. I have no direct experience with these types of crossovers and can't comment directly on how great the impact on sound quality may or may not be.
(2) Some companies make kit or assembled active crossovers (analog) that may suit your needs. The most well known (and the only one that comes to mind at the moment) is Marchand Electronics. They offer active crossovers of most designs, and the ability to select the crossover point/slope etc. before ordering. Some are made with a slot that can be interchanged for other slots (purchased separately) to "play" with various crossover points. I think they might even have versions that are tube-based. I don't know if they include level matching, however. The sound quality of the Marchand crossovers is reputed to be excellent, though I have no direct experience with these either.
(3) You can build your own active crossover using an available board layout. The most familiar to me, since I have used it in the past, is the board available from
Rod Elliot . This board is a Linkwitz-Riley design that can accomodate a two-way crossover of 12 or 24 db/oct, and also has provisions for signal level matching/control. When using components of respectable quality, I can attest to the sound quality of this crossover - when I've experimented and made the bandwidth of the passband extremely wide (essentially an all-pass), I could not determine when the crossover was in or out of the signal path. I believe this crossover to have excellent performance. The filter type however isn't flexible (AFAIK), nor is the slope (6db and 18db/oct are not available, AFAIK). If it still meets your needs though I would recommend taking a closer look, as the value and performance are excellent.