How flat should my room response be?

I am in the midst of treating my listening room. I also purchased an spl meter and a test tones cd. In a perfect world, we would all strive for a perfectly flat room response. However, given budget constraints and the reality of my room configuration, I know this is not possible. Given this, how flat should I try to make the room response? Is +- 2db ok? Or is +- 6db ok? I am looking for a realistic goal that I can set that will yield the best possible results.

Thank you,
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You will need to calm the worst resonances as a priority. If you can cut the amplitude of the room nodes in half, your ears will already tell you you're doing well. A lot of trial-and-error is involved here, since those nodes are located in space at different points in the room volume. If it were me, I wouldn't be worrying too much about the numbers during the process.
I really agree with Tobias, here. I used my TacT pre and spent 2 days "dialing in" flat response with a RS spl meter and then let the TacT dial in a flat response. Wow, did it suck! Unlistenably horrible. I'd serously rather have a Bose radio than my $50K plus system, if it usually sounded like that. And, as Tobias points out, move the listening couch back or forward a couple feet and you have a whole new set of issues/numbers due to physical locations of the nodes.

Tame the room nodes at your listening spot(Parc or TacT are the ones I'd use) and don't try to cram too big a speaker into the room unless you want to spend a ton on bass traps.

You certainly don't want a "flat" response in the upper frequencies, as this will kill the cat and the dog and cause you to run from the room with fingers in ears. The high frequencies should roll off gently and smoothly. You can do some web searches to see what this response curve would look like.
We consider a room flat if it's within +/- 3 db in general--this varies somewhat depending on size because the smaller the room the harder it is to achieve that--larger rooms can be better than that. If you have bumps at 5 db or greater that is probably the time to consider equalization if you've already employed bass trapping. 4 db--maybe, but typically not--it's a grey area. If you can't do bass trapping for domestic reasons (most are not that pretty unless you build them in), then you may need eqalization more readily than others because bass trapping helps smooth out the entire response--not just bring down the peak, when it's used right.

If you want to learn more about these issues there are a variety of articles in the Rives Resource Section of our website.
If you don't have any room correction equipment, then I would say +/-5dB, since that is what I shot for.

I find that the biggest improvements (without electronic or digital correction) are from a combination of speaker and chair placement. Often we as audiophile put both (chair and speakers) too close to room boundaries.
As flat as possible. There`s ZERO room for equivocation in the word FLAT.