It's like having a bottle-kneck in the whole system. The prob is that even if you solve the harmonics with convolving, you still have the issues of inability of the transistors to replicate the transient movements of tubes. These plugins are likely DSP gimmicks based on mathematical convolving the source signal with some type of Fourier transform, offset, from the original wave.
If anything it sounds too smooth and dead to me. I've tried many tube amp plugins and seen the harmonic distortion, but what comes out after an amp + pre-amp, that are solid-state, does not line up. You can bring up even or odd harmonics, or even to the 11th order but the bottom line is you are still going through transistors. It's like trying to reverse the very mechanism with DSP correction, it never works out.
Due to its flexibility, it can be used on every track or even as a mastering equalizer. Download free plugins for Winamp in Plugin Skins category, all Winamp plugins directory on. I do not believe any 'plugin' will be able to adjust the harmonics correctly to give the 'tube warmth'. Plugins > DSP/Effect Electri-Q Professional EQ for WinAmp and VST Professional EQ for WinAmp and VST Electri-Q is a simple, intuitive, but powerful minimum phase EQ, which makes it very easy to adjust the magnitude of any audio material. Spoken from someone who has a master's in Computer Engineering and has taken Image and DSP processing.
