I just purchased one, have had it for about 10 days now. I had been using an Accuphase E-380 integrated to drive my Spatial Audio M4 Sapphire open baffle speakers (4Ohm, 90db). I am now using the Accuphase E-380 as the preamp and connecting via RCA to the S5.5.
I love my Accuphase but the allure a a pure class A amp was too great not to try. Having listened to the S5.5 and done back to back comparisons withe the Accuphase I will say that the S5.5 does everything better than that Accuphase amp. It has much better control over the bass, and it is faster/tigher. The midrange is where the Accuphase shines in my opinion and I would say the S5.5 is equally as good. The upper range is where the S5.5 really pulls away from the Accuphase, it is much clearer, more detailed and airy. Not a bit of harshness and no listening fatigue.
The S5.5 has added so much detail and clarity to the music it is crazy. Everything is crystal clear and detailed yet the music just flows and sounds amazing, nothing harsh or clinical.
I know for 100% certain the S5.5 is staying in my system. The only question is do I sell the Accuphase and get a dedicated preamp or keep using the E-380 as a preamp only.
Highly recommend the S5.5!