If your primary focus were only jazz music, I'd say based on measurements I've seen that the Paradigm sub might be adequate. But given that concert bass drums often have a fundamental between 16 - 20Hz, and some organ music can contain 16Hz tones, I think your orchestral reproduction would strongly benefit from something not quite so vegan lifestyle. You know, depending on your budget, it might not be a bad idea to have Jim Salk design and build a pair of hidden-in-plain-sight furniture grade subwoofers for you. Might be worth giving him a
phone call or email.
Re: no processing, that seems to be another sort of vegan lifestyle choice. Room interactions will cause frequency response at your listening position to be quite a bit different from how your speakers would sound in an anechoic chamber or outdoors. Being able to correct the room response and to align your various speakers in the time domain are two big reasons to use an AVR or preprocessor of some sort. Another is an electronic crossover to high pass your mains and low pass your sub; although some would argue that 3 bass drivers are better than one. I say if your KEF's were up to the task, you wouldn't have started this thread in the first place.
Since preprocessors typically come at a premium, your best value will probably come from an AVR. If you dig your current amp, you could use an AVR as a preprocessor and keep using your amp to power your KEF's.
This refurb Marantz SR7008 would be a great choice. It will independently correct two subwoofers, offers preamp outs for all channels, and it has a very capable automated room correction for your mains as well.
