Voxengo - Sound with a Soul
Professional VST PlugIns and
AU PlugIns

I'll use Reaper for this tip

go to http://www.rs-met.com
in the freebies section download the Signal Analyzer and put it into your VST plugins folder.

Load up Reaper.  Assign the Signal Analyzer to the master bus, and float it's window by double clicking on it in the FX window

also assign Warmifier also to the master section, but before the Signal Analyzer.

create a new Virtual instrument track and assign the REAsynth VSTI to it.  Just create a sound with a pure sine wave, reduce the output volume a little so it doesn't clip.  Hook up a midi keyboard and hold down a low note.

note the shape in the Signal Analyzer
It's a pure sine wave when TapeBus is bypassed

Play around with the controls and note the changes visually on the Signal Analyzer and the sound .  It helps a lot

You can see and hear the major differences between the tube/valve types!  They all add varying amounts of upper harmonics.
Some are a lot smoother and cleaner sounding than others.  Others much harsher and fizzier.

Quite a difference between Mode 1 and 2 as well

Whoops I forgot you can use

Voxengo Span as well :)
http://www.voxengo.com/product/SPAN/