Subwoofers and "speaker level" (or "high level") inputs

Hello there. I'm going to add a subwoofer to my system. My recently bought integrated amp doesn't come with pre-outs, so I'm considering buying a sub with "speaker level" (or "high level") inputs and output, and thus unloading the speakers from low frequencies.
Someone advised against this idea on another forum, pointing out that the high pass filters are usually only 1st order, and usually have a cheap NPE capacitor at that, and they would "present a high impedance load to the tube amp below the high pass frequency".
I don't have any technical background whatsoever, so I'm trying to find out whether using those "speaker level" (or "high level") inputs and outputs would or wouldn't be a good idea. Two sub-questions:
a) I've read many times on the web this opinion: "the input impedance of the subwoofer amplifier is likely to be over 1kOhms. The amplifier feeding into it won't notice that it's there." If that is the case, I fail to understand why the "high filter" would imply a "high impedance load" to the tube amp...
b) Aren't there any subs that actually do a good job filtering frequencies through "high level" inputs? Do they all come with poor quality caps?
Thanks for any feedback on this one!