Speedy, I don't listen to 2-channel material using only front speakers when surrounds are available. As to the effect of using any type of "pure" or "direct" mode, the only effect that this can have is to slightly lower the noise floor of the amplifier. In most cases this is of no practical significance. The noise floor of typical receivers(including the three I have)is such that with the volume control at max and my ear a few inches from the speaker(no program playing, of course)only a faint hiss is heard on two of them(none on the third)which can't be heard either at normal volume settings or normal listening distances. So, in effect they're operating at a noise level below audibility already, and a further reduction is of no benefit. No other sound factors are affected.
