How Does a "HI-LEVEL IN" Affect Impedance Load?

It's a shame your Jolida SJ202a doesn't have line level "preamp output" to take to your subs line input, as I've always found that line level is tighter and more uncoloured for a sub than using the high level speaker output, even with SS amps.
I myself would put L&R rca preouts from the preamp circuit, on it if it were mine and I desperately needed subs

Cheers George 

There is no such thing as stereo sub bass. This is a hard one to understand but I will try and explain. Again. Really low bass frequencies are experienced very differently than higher frequencies. We cannot for example even hear a very low bass tone (30 Hz say) unless it is at least one full wave length. One wave at that frequency is far longer than your room. The front of the wave has hit three walls and the ceiling and bounced back again before the one wave even completes coming out of the speaker. So how on Earth are you gonna know where it came from? You’re not.

Also, all the really low bass is summed to mono.

But- and this is the hard part- this is NOT to say that low bass does not SEEM to be stereo. With a really good DBA the bass is just as seamlessly 3D as the rest of the sound stage. https://systems.audiogon.com/systems/8367 This is easily demonstrated by running a system like mine where the exact same setup can be run stereo or mono. It makes no difference. Sounds exactly the same.

So definitely buy 2 Sunfires. But do it because the more, the better. Forget everything you hear about stereo subs, stereo placement, timing (ha! did you read the above??!) and all of that. In fact, be sure and place them asymmetrically including relative to your L and R speakers.

Hmm. This certain local seller does have two Sunfire MK2's, for sale. I like the stereo option! 

That is how my two subs are hook-up, in stereo. I like it vs LFE.

Thank you all so much for the detailed information. It's hard to believe that in all these years, a guys has never used speaker-level inputs on a sub. The more you know... 

Right. No effect. This paper explains exactly what is going on. If you can find the circuit diagram for your sub you will find this same circuit inside the sub. The values of the resistors may be different to accommodate a wider range of amplifiers, and there will be a trim pot to adjust level, but its really the same basic circuit inside all of them.
https://www.epanorama.net/circuits/speaker_to_line.html

Sub speaker level inputs are usually in the range of hundreds to tens of thousands of Ohms.

So, doing the normal math for parallel resistance (which is close enough to impedance for us to illustrate) :

R = 1 / (1/8 + 1/10000)

where 8 is the speaker impedance, and 10,000 is the sub impedance, gives us 7.99 Ohms, and a difference of around 00.1%.

It’s insignificant to a power amp.

Not much, if any at all. This is how I have my sub amps connected. I do recommend, however, the same type and size wire, as to your mains, if at all possible, for the best sq, based on my experience. Enjoy !, and be well.