Thanks for the feedback on this.
I know what you mean about 12AT7/ECC81 liking to oscillate, ive heard a bit about this before.
I might look for another design elsewhere I can use instead, I quite liked the idea of a by-passable tone stack, hence why I was interested in it.
I see he has a revised version of this schematic here:
http://www.dogstar.dantimax.dk/tubestuf/instpre.htm