Crossovers, why so few options?

Just to clarify, I'm talking about using an external crossover to split the signal between sub/mains.

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.

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...

Does anyone here use an active crossover with a tube preamp? It seems like the input impedance on most active crossovers is pretty low as compared to tube/SS amps; has this been an issue for you in matching the preamp to the crossover?

I actually emailed HNT, they told me the input impedance for the X2 is 10k ohms (seems rather low). I don't see how this wouldn't cause a problem with many tube preamps.