A currently produced high quality passive sub I am aware of is the Daedalus BOW. However it is definitely not inexpensive, and it has presumably been designed with the expectation that it would be used with Daedalus speakers.
the terminals are high level inputs which you connect to your stereo amp.
It doesnt bypass the sub amp. but you could always change the sub amp or convert to passive.
I dont know how much EQ if any is applied to these active subs. If the response is equalised then you cant remove the class d amp and connect a different amp and expect the same response.
I believe that you can get passive versions of the Bag End subs.
I assume you have an extra amp laying around that you're not using. That's probably not the case for most folks, thus, a good reason that active subs are so common. Besides, there's no chance of amp/speaker mismatch with an active design.
I build a subwoofer system called The Swarm that consists of four small passive subs and a separate amplifier that drives all four. On occasion I have sold the subs without the amp, but I don't sell the subs in ones and twos - only four at a time.
Imo there are two significant advantages to not having amps inside the subs. First, in this day and age of lead-free solder, electronic components like subwoofer amps are significantly more susceptible to failure if they are subjected to vibrations. Second, if the amp does fail, it's much easier to repair or replace an outboard amp than to repair or replace an amp that's built into a sub.
Parts Express sells subwoofer boxes that you can install a driver in and end up with a pretty darn good sealed-box passive subwoofer.
Anthony Gallo offers passive sub woofers intended to be used with their line of separate reference amplifier/crossover/EQ. I would recommend those. They are also bridgeable which is nice for versatility if you choose to go with a single sub.
Another manufacturer that offered separate sub amps and passive sub woofers is NHT. Not up to par with the Anthony Gallo offerings, but decent you can have some fun with them.
Dayton has both component sub amps built into a case that can reside on your rack with your other separates. They also have a few different plate amplifiers, with and without an EQ.
If your going to employ a plate with or without EQ personally I would spend more and get one from Hypex, one of the leading manufacturers of class D systems.
In the past I employed a 2ch A/B Bryston 4B with mini DSP and had pleasant results.
As mentioned some of the ready made to order subs kits that are out there can be a good way to go, with and without electronics. There are some well made sturdily braced cabinets offered with and without sub woofers or plate. Do it yourself sound group is one place I would recommend getting them. You basically just have to assemble your box they take some of the guess work out. But you will have to find a suitable sub woofer driver to match with the pre cut cabinet perimeters for optimum results. So some homework is involved there. Or if you feel more adventurous some sheets of your choice material and a table saw can be rewarding.