a huge number of misinformed people here believe EQ to be the ultimate devil(most without any real knowledge or experience of a good EQ). but doing exactly the same **** with at least the same defects by looking for a "colored amp" is OK.
didn't you ever hear something like "EQ is destroying the sound, I want the real sound"?
Depends on the EQ. An electronic device is characterizable by its frequence
and phase response. The phase response is, effectively, a time delay that varies by frequency. A 'flat' phase reponse is effectively a propagation delay. Different analog filter designs (butterworth, chebychev etc) have different characteristic phase responses. To a large extent good analog filter design is about trading off undesirable phase response in favor of desirable frequency response. The problem with non-flat phase response is that it changes the positioning of the frequency spectrum in the soundstage. Your filter may attenuate around 1kHz, but it then causes 2kHz to show up later. (In the time domain this shows as ringing. All filters ring.) This is in analog filters. In a DIGITAL filter, a so-called sinc filter, phase response is flat. Always. There is a propagation delay equal to the size of the filter kernel (5, 11, 64 samples, or whatever). This is obvious to anyone well-versed in the art - convolution with a kernel of size N requires holding onto a window of N samples. Phase response is flat at N. Straight line.
The way you characterize an electronic device, typically, is by feeding it a unity impulse. This is a brief spike (as brief as possible, less than one half sample). You then sample the output. From this you can create a filter where if you feed it the output you can deconvolve it into the input. It works for any signal and effectively any device. Through a complex Fourier transform of the collected data you get the frequency
and phase response of the device. It can be graphed and used as effectively as any sweep or analog method (except it avoids some phase problems in instrument amplifiers and such). Similarly, a filter kernel can (easily) be produced that takes an input and replicated the behavior on the output, effectively emulating any device.
Well, any LINEAR device. If you feed it a half-unity spike and get a
different response you have a non-linearity. But different I mean a
different waveform out, not just a weaker version of the unity response. The bad news is the real, physical world is non-linear and we makes simplifications where we approximate it to be linear. Some things are close enough to linear that the error is irrelevant, such as transistors. Tubes work by electrons beaming from a cathode, gated by a grid, and collected onto an anode where they form the output signal. This is not a very linear process. Because tubes aren't linear a characterization in terms of unity frequency and phase response they can't easily be characterized, and the physical mechanism is too complex to easily produce a mathematically accurate model. It's truly woo.
So "coloring" can mean many different things. It can be a digital EQ, an analog EQ, complex impedance (i.e. reactive load), the response of a linear device, or the response of a non-linear device. For the non-linear kind, the only really practical approach is to swap things out and see what happens.
Non-linearities also come in many different kinds, with different subjective results. Some produce ugly distortion. Some, not so ugly.