Hi. I'm currently in the middle of changing the OpAmp in my VTB1 from a 4580 to an OPA 2134. I'm intrigued to hear whether it would make any noticable difference to the sound.
One of my colleagues is an electronics technician and is going to do the job for me.
Is there anything else I could do to the VTB1 (within reason.....the OPA2134 costs £2) that would improve the sound of this pre?

I have not. Allegedly, Ted Fletcher, formerly the man behind Joemeek and now the man behind tfpro, designed this mic pre. I believe he's got a forum at the tfpro website. You might ask him what he thinks.

Otherwise, you might pop over to the forums at PMI Audio.