What Rex is getting at, is when you play your guitar in a live setting FRFR or CAB the sound couples into the guitar and resonates your guitar, this adds depth to the sound. When you record direct you lose out on this resonating effect and the tone can sound lifeless and sterile. A lot of people forget about this and when they record direct they wonder why the Axefx doesn't sound like a mic'd up real amp. If you are looking for a quiet route there has been experimentation with a vibrating device I forget what post it was talked about on.
Here's what Cliff had to say:
It's not from the "tubes being pushed". It's due to sympathetic vibration of the guitar itself.
AFD sounds great because it was recorded loud. In fact, Slash had a Boogie combo in the control room pointed right at his guitar. He had a volume pedal controlling it so that he could adjust how much sustain/feedback he got at will.
When you play at low volumes there is little sound coupled into the guitar body and strings. As you turn up the volume the sound couples into the guitar and reinforces certain frequencies and increases the sustain.
I did a test some time ago and the change in the frequency response is quite dramatic. IIRC, I measured over 10 dB difference at certain frequencies. This is huge. The reason I was doing the test is I was trying to see if I could recreate the effect of playing loud but at low volumes.
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