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!
I have a couple of RELs from the "Q" model era and they work as advertised providing seemingly accurate low end for days, and are built very well so if there are less than high quality caps in these things this is the first I've heard about that. In any case, the impedance "load to the amp" is so small due to the fact that it's simply using the speaker output as a very high impedance signal and not to drive anything…I have my 2 RELs on the same speaker posts as the main speakers and there is zero audible or loading effect on the power amp for those reasons…the general tone of the power amp is there (low range only of course). You adjust the output and range of the sub (phase also) to fit your taste with no effect on the main speakers except to generally improve how they sound. Note, if I was buying a new REL I'd get that "longbow" wireless gizmo as the you can more easily put the damn woofer anywhere to get the best sound.
Fedocable it looks to me like.... The best way for you to add a sub is by using a REL because then you can purchase a upgraded SPKON cable from a company like Signal Cable. The SPKON cable is a special connection that REL sub woofers use to connect using your amplifiers output taps! Also, REL allows for a wide range of adjustments to blend and become part of your main speakers. The cable and the way it connects are way different then using RCA’s from a preamp, or trying to feed it a (high power signal) from your amp. I have a REL and I have a second set of main outs on my pre amp that I could use for the REL (I dont). I use the SPKON cable connection from the back of my REL to the amps out puts, because this is the best way to hook it up sonically, period. So your all good :-) forget about wireless, IMO.

Matt M