Crossovers, why so few options?

Don't most stereo preamps not have a built-in crossover?

Additionally, outputs from subwoofers usually have a high-pass filter that isn't adjustable; I know that on REL subs, this is as high as 120 Hz for line-level outputs. Most people (I would think) don't want to use such a high crossover frequency.

the velodyne sms1 offers a lot of flexibility & EQ for sub/main use, couple of products in that area of sub x-over & EQ. also has a remote to fine tune to different recordings

Marchand Electronics has external crossovers

I'll mention that even if subs provide bass management, there are some problems with this arrangement. Suppose you have a tube preamp and stereo subs. The input impedance of sub amps is usually pretty low (~10k ohms), and with 2 amps it works out to 5k ohms. This makes it pretty problematic to run line-level outputs from preamp to sub amps.

In addition, if you wanted to run speaker-level signal through the sub to the mains, the sub's high-pass filter frequency/slope is not adjustable without a hardware change.

It seems like the only solution is an external crossover...