AVS Special Member
Join Date: Dec 2007
Location: Southern California
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The KEF speakers will be a HUGE sonic upgrade, but the problem is that they are basically a 4 ohm speaker, which the receiver is not designed for. The difference in sensitivity is not the main issue.
If you look at the impedance vs frequency curve for the Q300, they do have a 4 ohm impedance at many frequencies. Not good.(the KEF 8 ohm rating is incorrect and misleading).
I suggest that you look at the DALI ZENSOR 1 speakers, which are an easier load to drive and are excellent speakers. They stay above 6 ohms over most of their range and Stereophile measured their actual sensitivity as 88 db per watt in their recent review.
Amazon has them for $475 in black or $525 in a light walnut finish.
If the receiver gets hot enough that you cannot put your palm flat on the top panel it and keep it there for 10 seconds or more, then it is too hot, and you may want to look at improving its ventilation. Check the actual receiver chassis vent hole locations and make sure they have 1 inch of free space above or below them. Putting something about one-half inch thick under each of the feet and raising it may help.
Also, if it is in a cabinet with a back panel, make sure there are plenty of holes in the back panel. If not, cut some holes with a hole saw, especially at the top of the space the unit is in.