I think the wood and the bridge make up the difference in tone as well - it isn't all in the preamp. All maple body wings, pau ferro fretboard (which produces a lot more growl than the rosewood in the Euro models), solid brass (and HEAVY) bridge... these contribute a ton to the tone beyond what just the pre amp is going to do.
I had it done on my old black Ian Hill....it improved the sound (slightly).....I would say before the preamp change the sound was a 7.5 (1-10) after it became an 8. So basically if you have money and don't know what to do with it go for it....It won't make you say "WOW" this sounds just like a NS2!!...It will improve slightly, but is it worth $350????....Like I said if you have money....I might actually get it so I don't have to change the settings live on my head gain volume. The euro tone pump has to be lowered big time compared to the others. Since I will have 3 basses with the US 9 Volt (formally the HAZ, hehe) I might make them all the same but I know it won't make the euro sound like the others.

