Those Ion amps weren't very good I remember - sorry.. The legacy of the lively but open Nytech era amps had given way to a hard toned and rather harsh kind of sound engineered in from new and we couldn't sell them at the time.
I sold and loved the S8 when new and can sum them up as a refined and smooth performer, needing a bit of power to get them going and maybe with a preference for silver plated speaker cables (we sold Chord odyssey by the km back then for speakers like this). Apart from that, I wouldn't say the S6 or S8 had much of an 'attitude' in all honesty, unlike the smaller S5e or larger S9 (both versions).
Can you borrow a bigger amp? My old friend who gave me his then knackered Bc2's (which I've since repaired to full working order) went for some S8's and loves them. He uses a Musical Fidelity amp from the early noughties - the large integrated with a trim ring (lots of 'bolts' around the volume knob) and finds them a good match. of todays affordable amps I'd probably suggest an Emotiva if they still do the integrated, or one of the better Japanese integrateds by Pioneer or Yamaha - something with a bit of 'quality push' to the output. I suggest a perennial favourite such as a Rega or Croft would get a little lost when the volume goes up... Silly money for what they are now, but the Naim separates 'sounded' punchy and dlively with the S8's - but not the underpowered Nait 5 series, as they hard clip as I suspect your amp is doing?
Sorry if I have it wrong. This series of Spendor are very good if matched right, but they need a helping hand power wise to get them to really disappear from the proceedings.
I can't go on and I can't go back, I don't feel so matter of fact,
I tried and tried to make good sense, What's the good to try it all again?