I wanted to say a few things about the new Speaker Breakup feature in Eleven Rack, so all of you can get the most out of it. My personal and very biased opinion is that it's a fantastic feature, which makes Eleven Rack sound even better, so it's definitely worth checking out. Also, the algorithm has been totally redesigned from what's in the Eleven Plug-In and has a completely different control taper, so I suggest discarding any preconceived notions. But regardless, I think you'll like the new one even better.
When you first start playing around with speaker breakup I recommend using headphones, because usually the difference is a little more noticeable. While any preset will work just fine, I think some presets do a better job of showing off the difference that breakup makes. Bolder Axe and Browntown are particularly good choices.
Under the hood the breakup control is mapped to numerous parameters, so that the user can get a considerable range of sounds from a single control. The speaker breakup algorithm is a combination of cone breakup, speaker "compression" and other types of speaker distortion. In a very simplified sense the breakup control adjusts the amount of breakup distortion and it also adjusts the amount of natural speaker "compression". The speaker compression is not really like any compressor effect you're used to, so I used the term "compression" loosely. Anyhow, it mainly affects high frequencies, so the sound tends to get darker as this compression is increased. Sometimes it can also sound as if there is less breakup when the sound gets darker from this compression, even though the actual amount breakup distortion may have increased.
Around 1.0 to 2.0 on the dial the algorithm adds a relatively subtle "smoothing" of the high-end, although even this setting can be quite obvious on certain material. By about 3.0, speaker compression kicks in fully, which results in a somewhat darker sound. At about 5.0 there is less compression, but more breakup distortion. Above 8.0, the speaker breakup distortion starts to become more extreme than what would typically occur in an actual guitar speaker. These higher settings tend to work better with darker-sounding cabs. And as a general rule high gain amps work better with lower breakup settings, while low-to-medium-gain amps often work well with higher breakup settings.
More specifically there are a number of nice effects that breakup can have on your sound. For one, the algorithm only kicks in when you are pushing the amp and speaker really hard, so it will have virtually no effect on purely clean tones or on distorted tones with the master volume turned way down. But if you set the amp right you can get nice clean tones with your volume backed off and raging distortion and breakup with the volume maxed. In other words, it can make the sound more expressive and responsive.
Also, it makes the transition from clean to distorted smoother and more gradual. Without it there is a clearer transition at the point when the power tubes start clipping. This can be a big benefit for moderately clean tones, because transients that clip don't stick out as being obviously distorted, but instead sound "smoother". In other words, without breakup the harder you play you get: clean, clean, clean, clip, clip Whereas with breakup as you play harder you get: clean, clean-ish, slight clip, more clip, lots of clip.
At certain settings (e.g. 3.0) the speaker compression dominates, so generally you get a darker sound the harder you play. This can smooth transients in a nice way. Of course, it will depend on the particular application and what you're going for.
And last but not least, it can smooth out high-end "fizziness" from tube clipping. Even tubes can hard clip if you drive them hard enough, which results in very high frequency harmonics. To me these very high harmonics sometimes sound as if they are separate from, or sit on top of, the main guitar sound. With breakup I would describe the sound as being more integrated. In other words, the low frequencies and the high frequencies better blend together to create a unified sound.
Ultimately I think it's more realistic, but I'll let all of you be the judge of that.