PEQ filters how sparingly should they be used?

I have a two part question. One is "what Behringer and other manufacturers are using as digital filters on their PEQ's" and the other is about "what settings to avoid ringing artifacts from applying filters?".

I use the Behringer Feedback Destroyer pro to make PEQ adjustments in the bass (below 50 Hz for room modes). I also see that others use PARCs, TacT and the Behringer Ultracurve Pro 2496, all for similar reasons.

I am not clear on what type digitial filters are used in the Behringer? Anyone know what they are and how they work? Details on other digital filters are also welcome (are there optimum solutions out there?)

Digital has the possibility to use linear phase filters, unlike analog EQ's which must use IIR (Infinite impulse response) type filters (digital can use IIR filters too, so it is not a given that the Behringer or any digital filter is linear phase, btw).

Digital linear phase has the significant advantage of not changing the phase at the same time as you make gain or Q adjustments to iron out a modal bump (For a specific listening position, I guess you can use the PEQ to nail that bump without another one popping up somewhere else due to an added phase shift!)

Nevertheless, both analog and digital will risk to suffer from ringing, especially if the filter is deep and very narrow; in the case of a Digital linear phase implementation, the ringing may occur before the primary signal, a kind of pre-echo. In the case of IIR filters the ringing will appear later and superimposed on a larger signal (some may say this is preferable).

I have played with the Behringer with Q's as narrow as 0.2 octaves and filter depths as high as -8 db, and although I do not hear anything I would call ringing or pre-echo I am concerned. (Am I fixing one room modal issue but potentially creating another separate issue?)

Anyone have an idea on what are safe settings for Q and filter depth?; settings that will not induce audible ringing.

I expect those who favor room treatments over PEQ's will chirp up, perhaps I can prempt this by saying that I totally agree that room treatment is the first and foremost solution (but it is expensive and very WAF unfriendly)

More details: I read somewhere that the Behringer Feedback Destroyer Pro has a latency of 1 Msec across all frequencies. This leads me to suspect that the digital filter is not a linear phase implementation for the PEQ, as there is not enough time delay to have applied a linear phase filter. I may sound confused here but I like to undertsand what my equipment is doing and their is precious little info in the manuals. I don't have a problem with my Behringer, in fact it works just great for sorting out room modes and for the price it is hard to beat....nevertheless I want to understand what it is doing.
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