Potential bug: When I play through either the Legend 100 I or II, these three speaker impedance curves cause the sound to muffle completely with a low frequency sound:
1x12 Dirty Shirley EV12L
I'm wrong. I'm just wrong. Here's the skinny:
So I looked into this. I'm pretty sure the real amp would do the same thing. What is happening is the power amp is oscillating at that very low frequency cabinet resonance. This occurs because the Legacy has so much negative feedback (12 dB, which is the highest of any amp I've ever modeled). Combined with the smallish output transformer there is a loss of phase margin at low frequencies which causes the power amp to become unstable.
The Rumble cab is unique in that it is ported and has two different types of speakers in it. This causes a double resonance and one of the resonances is at a very low frequency. For typical power amps with reasonable amounts of feedback this isn't an issue.
You can stabilize the power amp by reducing the Transformer LF. This will improve the phase margin and increase stability. Another solution is to reduce Cabinet Resonance but that will change the tone a bit.
Whenever you have negative feedback you run the risk of instability. Highpass and lowpass filters introduce phase shift which reduces phase margin. Combine that with a strong resonance and the system can become unstable.
What I found is that lowering the Transformer LF works with the Rumble 4x12, but with the Dirty Shirley, 1x12 USA and HiWatt, you need to raise it.