line level crossover for subwoofer

Hello, I’m looking for recommendations for a line level active crossover. I plan to use it in conjunction with a low watt tube amp and subwoofer. My 300B amp only puts out around 4 clean watts. I want to eliminate the low frequencies to the amp below around 150 HZ and have a good sub do the heavy lifting. I guess I would need a separate preamp as well in order to adjust the volume of both amps. Any thoughts?

Thanks