is it qood idea to use an Equalizer with amp

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

if most of your recordings are bad.

I disagree with you Elizabeth... have you actually tried all the various kinds of equalizers you've mentioned in a "high-end" system?

Have you owned a unit with advanced digital room correction like the Lyngdorf or DEQ units? Have you used the Behringer DEQ2496 full-range in your system? And have you tried a unit like a basic DBX 1231 in balanced mode in a high-end system?

I can say that in my system the dbx 1231 sounded very clean, low-noise, and musical with no apparent grunge. Playing the system in my large room without EQ makes no sense. The dbx only costs about $350 and will work very well for those folks who just require a modicum of EQ and don't have the funds for something like a Lyngdorf, which works even better but costs much more. I've not used the Behringer DEQ2496. I've heard some say it's good for bass EQ and for its built-in RTA, but maybe not great for a full-range EQ. But I haven't heard it with my own ears in my system so I can't confirm that.

I'd be curious to hear of your actual use of the EQ's you've mentioned in your own system, because your conclusions/assessments don't align with my experience.

Elizabeth, in retrospect I do agree with you on one point... the fewer A-D and D-A conversions you have in the signal path the more transparent the sound will be.

Frank