is it qood idea to use an Equalizer with amp

It depends on how bad your room acoustics are. From the sound of things you have little or no experience using an equalizer and you kind of have to know what you're doing in order to get good results.

But I have used both a relatively inexpensive dbx 31-band graphic equalizer and a Lyngdorf digital preamp with digital room correction and have gotten very good results. The system sounds much better with the EQ than without. That said, as I mentioned above, there is a learning curve--also, every aspect of sound quality cannot be fixed with an equalizer.

Let your ears answer the question. All in audio is about the ears. If it sounds better, it is for you and that is all the really counts. Don't let 'old wife's' tales rule.

If the equalization is done to a digital stream, from a Cd transport, and before it is decoded, it can be pretty clean.
If the analog signal is digitalized and decoded just to EQ it, maybe not so great.
For a CD setup where you have a DAC, then adding in a device like the Behringer DEQ2496 and adjusting the EQ, probably would do no harm, and could offer benefits.
For a standard mid fi system, an analog EQ device might be acceptable. For most high end systems, an analog EQ device would add far to much grunge to be acceptable

I have an EQ, Luxman G11 - completely recapped and brought back to spec. This was a $500 EQ in the day. I use it with my Cary tube preamp, Monarchy amps, Usher speakers. I have it connected to my system using a home brew selector box (like a tape loop) so I can remove it from the circuit entirely. Like using it for some poorly mixed material or live recordings where the bass and/or highs are rolled.
Even with the EQ in the circuit and set to neutral settings - very hard to detect. So, in my opinion they can be used in a high system but best have a set up where it can be isolated from the signal path