Depends on a few things
-Quality of the load box
-Quality of the solidstate power amp
-Sensitivity of the tube amp to impedance differences
not all load boxes accurately replicate the impedance curve Of a speaker cab. A speaker cab will have a large resonant peak somewhere in the lows of maybe 40 to 100 ohms or more, then a valley in the midrange and then slope upwards as frequency increases due to voice coil inductance.
See the ‘speaker’ page in the axe FX for the impedance curve that the amp model is ‘seeing’.
replicating that low resonant peak is expensive since it requires a big inductor of very precise manufacture, so some of the cheaper load boxes/attenuators skip it, or simulate it in a different, less accurate way.
Of the impedance plots I’ve seen the Suhr is the most accurate. I haven’t seen any plots of the fractal x-load, but given the size, price, and cliffs propensity to not cut corners, I assume it’s also accurate.
here are some plots of a cab, followed by various load boxes
http://www.masterplant.com/electronics/Z.pdf
Clearly the OX and captor are not that accurate but how much does it matter?
that depends on the sensitivity of your amp to the impedence curve. Generally the less negative feedback in the power amp, the more sensitive it is. I’ve tried resistive loads and attenuators, and resistive loads in parallel with a speaker cab with my Mesa Rectifier, and in Vintage mode I can get away with more attenuation before the sound is compromised, where in Modern mode (negative feedback loop disconnected) any flattening of the impedance curve at all greatly kills the tone.
finally, even with a great load, if you use a solidstate amp to reamp that is no good or has compromised frequency response, your results will suffer. A quality amp is important.
there are units designed to do this now, such as the Waza TAE, and Fryette powerstation.
alternatively if you just want to record a signal from the amp direct though impulses, you can use a DI box (with a speaker-thru that can take speaker level) between the amp and cab, which will tap off a low level signal from the speaker tap to feed into the axe FX. Alternatively if your amp has a slave out or line out, that’s probably what that is.
