Another Sub Q: Is my sub mismatched with speakers?

beside, all the points above, dont forget that many speakers claim to reproduce music in low Hz, but they dont sound as good as quailty sub,, the best is to do comparision and hear, even cross at 125 Hz , may sound more solid and richer bass

Are you using the crossover in the sub? Does your preamp have a crossover built in? If you are running full range to your speakers and full range to the sub, you will get some overlap with a crossover at 50hz. Remember that most crossovers are not 1st order, so you will get a gradual roll-off over a 12db to 18db range. The only way to avoid this is to have the system crossed over between the pre and power amps or between the amp and the speakers so the sub won't get too much of the same signal.
Some very high end subs will allow you to adjust for a good blend.

I think that if you run your main speakers full range you have to set the sub crossover below the mains. So I think 30Hz is a fair guess for where you would have to cross.
My guess is that the crossover at 50Hz will result in boomy, muddy bass. It's pretty clearly audible if there's too much overlap, and it doesn't sound good.

As I see it you have two choices ... either put a high pass filter on the main speakers to take them to about 60Hz cutoff (I think that ACI, makers of the Titan sub sell passive filters for this purpose). Another filter option is to get an active crossover, but these can be expensive.

The second option is to get a new sub like a REL which can crossover at 30Hz or below. REL goes to 22Hz as its lowest crossover frequency, and I think this is a key reason for REL works so well with so many speakers.