It doesn't sound like anything a tube would do. There may be some sort of noise gate built into the circuit? Maybe the bias is hinky on one of the opamps? A lot of silicootie tech is beyond me though. Here's a link to the schematic to make it easier for others that might take a look.
http://bmamps.com/Schematics/vox/Vox...2005)%20SM.pdf
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