well done lords, I have one of these amps, and really rate it, the build is first rate.
Quite right it is a bit bright for a valve amp, but you do get used to it.
For speakers, its not really the lowness of the load, ie 4 ohms that is significant, it needs not to veer around too much.
Proacs suit this amp extremely well indeed.
t-bone, all correct, I am a bit of a wad expert, earlier wads were a bit unreliable due to poor resistors, but pretty much all were designed by A.G. the circuitry is first rate, I highly recommend all 2 watt resistors as a basic minimum for reliability and chunkiness whereever. Interesting in the icon/ming all resistors are quality, at least 2 watts +. great sign.
6550 is a great tube, airy and addictive, but can sound a bit thin and transistory at first, kt88s are more ballsy. el34 bit of both. it depends massively on the amp as well, in the icon, different tubes didn't sound massively different to me.
the icon amp is grid bias, so you can vary tubes if you know what you are doing, but it takes a bit of calculating...the volts are about 410-430v on both anode and screen grid. FWIW, the icon uses quite a 'cool' bias of around 50% of the valves max rating, I have found to hot causes a thick colouration to the sound, and shortens valve life considerably. I am currently of the opinion class A operation is unnecessary.
transformer changes are a pain in the ass they are like coupling caps in that they differ, but all have some attractive qualities. the icon ones are particularly well made
wads now are either cathode or a combination of the two!!
I don't think most 300b amps are worth it for the price of valves, the circuit has to be right otherwise they can be weedy, its uniqueness arises as a result it can be used without overall feedback, along with a handful of other valves, the effect of this is quite astounding. it makes other amps sound flat, much like going from a transistor to a valve in the first place.
there are only a handful to my knowledge of seriously good 300b amps, one of which I am fortunate to own.
A valve rectifier has a colossal effect on the sound, I tried both in the same amp, suprising, as its just dc, and it wasn't all about bass, the valve was far more airy and nicer, single ended tubes are particularly fussy about the power supply