After being dissatified for a while with the gain on my Burman Pro501, I decided to replace the remainder of the chinese valves left in by the previous owner (from Lindisfarne) with Sovtek 12AX7 LPS valves.
All I can say is that it transformed the amp overdrive from a cheesy sounding 70's fuzz box with a flabby bottom end from a Dave Stewart track to a nasty, raw sounding monster with a tight bottom end and loads of crunch.
If you want to bump the gain up in your amp, I can't recommend the Sovtek LPS valves enough.