sub bass active low pass cross/volume control?

1/(2Pi(sqrootLC) for a two pole LC LPF.. series L, shunting C.

There are also plenty of calculators online for multiple poled filters, which i suggest at least 4th order.

The hard part is finding a point where your sub's upper frequencies and your monitor's lower frequencies won't overlap, or else you'll hit problems with phase and mixing, frequency peaking or nulling. What you are doing is making a 3 way speaker, just with the low freq driver as a separate unit. You'll still need the equivalent of a 3way crossover, but instead of being able to do simple frequency sweeps with the speaker in an anechoic chamber to tune the crossover, you'll have to include the room as part of the sweep measurement, but that also brings in the question of room tuning, which is much more important at sub frequencies.

But then again, if it's just for fun, there are two ways to do this. Filter and volume before the amp, inline with the input, or on the output, between the amp and speaker. It's up to you to determine which is better, but the cheaper solution is before the amp.