Hi R&R:
Maybe I asked the question, I dont recall, but I am already aware of the fact that just about all receivers cannot switch more than two pairs of speakers at the same time, because every pair thats hooked up would halve the impedance. So adding a 3rd pair of 8 ohm spkrs would result in the impedance dropping below 4 ohms at which point the receiver in question would begin to feel the "strain"- however, the degree to which this happens is variable since impedance is not a fixed value, it varies higher and lower over the audible freq range, and each speaker has its own unique impedance curve- so it also depends on which speakers you have connected , & other factors come into play - taking into account how the particular receiver is built- how hefty its power supply is, how many pairs of power output transistors etc..
You mentioned that your 9900Z gets quite warm. Does it stay that way after you stop playing music thru it and its just idling? Its quite possible you might need to check the dc offset and adjust the bias cuurent- see EW's post in the other forum, the General forum, I believe it is, or perhaps Vintage SS, it explains the procedure. I dont think the fan should come on unless the unit is really being driven hard. My 300W BA-5000 power amp, for example, has a fan but it takes a temperature of almost 160 degress F before the fan should activate.
B/F
