Contrary to the above I will state that suitable speakers need not be exceedingly sensitive at all, although as with ANY low powered amp, it is usually always a bonus.
I have used this amp with many speakers including: Martin Logan esl's (~87dB) with stunning results, completely to the astonishment of all present, had great results with Gale 401's (86dB) which historically require Krell levels of current delivery, Meadowlark Kestrel II's (~89dB) and Quad 57's (~86dB) both of which sound excellent.
The worst combination was with Tannoy DMT 15 II which have a claimed 98dB sensitivity !
I've also used these in recording studios and they really need some serious grunt to control the huge heavy cone, with the 859/69 bass was almost non existent.
Search my posts on this remarkable amp, I've written several things pertaining to it.
Although Tim told me there was a marginal difference between the 859 and 869, when I upgraded my 859 to the 869 board, I also made some key power supply and coupling cap mods which vastly increased the performance gap between between the two (again check my posts), when tweaked this amp is right up there with the best of the best and I personally think that its soul sister which truly lets you get into the aural "Twilight Zone" is the Quad 57 !
These two are a match made in heaven and not surprisingly as Tim used the Quads as a reference point in the amps design.
