the au717 is a great integrated amp. competitive with expensive new product available today, especially if refurb'd and updated. (and the tu717 is a fine tuner as well.)
here's what i'd recommend, w/this amp. as it has preamp outputs and main inputs, it's a perfect amp to run with a good active outboard 24db/octave crossover and a pair of subs. as the l100 speaker's bass-midrange driver x-over is at 450hz, this means the bass driver plays well into the midrange. if you hi-pass the jbl's at ~60-80hz, they and the amp driving them will be so much happier, and so will your ears. just run the sansui's preamp outs into the inputs of a good outboard active crossover. feed the x-over's low-pass, set at 60-80hz (you can experiment) to your subwoofers. definitely use two; it offers so much better performance than a single sub, for several reasons. each sub will be working about half as hard for the same spl, which means less distortion and tighter bass. soundstaging will be better if the subs are positioned adjacent to the main speakers. and bass from two sources will mean less issues with room nodes. then, from the x-over's high pass outs, connect them back to the sansui's main amp inputs to drive the jbl's.
while i use a dedicated preamp and separate amps, i have been running all iterations of my setup in this fashion since the early 1990's. it's definitely the way to go, imo. and when i 1st did this a bit over 30 years ago, my main speakers were -2db at 20hz, w/crossover frequencies very similar to the jbl's you have. and yes, everything was greatly improved; the main speakers as well as the low end. while i use passive subs. w/active subs, just set their internal x-over at the highest setting; crossing over w/the outboard active x-over at 60-80hz will effectively bypass the powered subs internal x-overs. and some active subs (not familiar w/your specific model) have a direct input that bypasses the internal x-over altogether
you can get a decent inexpensive dbx x-over; even new, it would be <$300. but i'd recommend keeping an eye out for a used marchand or sublime acoustic unit.
doug s.
