I've done some quick testing using ecasound, and some of the LADSPA plugins, especially the 'preamp', and 'unmatched' plugins. The preamp plugin is designed after a Fender 5F4 preamp stage, and the unmatched plugin is "... a simple effort to emulate certain aspects of the tone of a real musical instrument amplifier, in real time". I really like the sound that these plugins create, especially on clean guitar.