comb filter

How do you avoid comb filtering effects where there are singers or instruments with microphones spaced closely together? Often in situations with limited performance space, it is difficult to adhere to the 3:1 rule.

Comb filter - A distortion produced by combining an acoustical (or electrical) signal with a delayed replica of itself (offset in time). The result is constructive and destructive interference that results in peaks and nulls being introduced into the frequency response. This response, when plotted to a linear frequency scale, resembles a comb (teeth) rather than a smooth curve.

In acoustics, comb filtering can arise in some unwanted ways. For instance, when two loudspeakers are playing the same signal at different distances from the listener, there is a comb filtering effect on the signal. In any enclosed space, listeners hear a mixture of direct sound and reflected sound. Because the reflected sound takes a longer path, it constitutes a delayed version of the direct sound and a comb filter is created where the two combine at the listener.

The frequency response caused by combining a sound with its delayed replica. The frequency response has a series of peaks and dips caused by phase interference. The peaks and dips resemble the teeth of a comb.

I just purchased a (used) "Studio Technologies AN-2" comb filter to simulate stereo from a mono source.
seen here:
http://www.studio-tech.com/PDFFiles/AN2ug_3.pdf

Do I need to be carefull when adding additional processing/effects downstream of the filter ?. .to avoid problems resulting if the "new" stereo signal is ever summed back to mono (radio/tv broadcasts). i.e. "apply additional processing/effects in equal amounts(?). and e.q. upstream of the (comb) filter"

OK, this may sound stupid, but I was wondering if anyone could help me with this problem and lend some advice. It's another phase problem. In a situation when I mic up a guitar amp and choose to use a close mic, about 5" from the cone and a distance microphone 1-3m away, I find that I experience comb filtering as I draw the 'distant' mic back and forward (which makes sense) but I can't seem to find a 'phase free' position? Whats going on. The comb filtering sweeps across the spectrum as you move the distant mic backward and forward, so how do you find a position with no phaseing? HELP!