Want to know if the tube in the Z2 makes a difference? The answer is: Yes. Today I spent some time swapping tubes. I waited about 6 months (per Alf's suggestion) to make sure the original tube was burned-in and that I knew what it was capbable of. The tubes that I tried included a bunch of new production and NOS 12AX7's, 12AT7 and 12AY7. The NOS tubes were excellent mid-60's examples of these tubes. First off, I was quite surprised how much of a difference the tube makes in the Z2. The stock Mullard reissue is not bad at all. The collection I tried in the Z2 brought out a bunch of differenct voices and and a range of feel/response. Out of this bunch, a new production Tung-Sol 12AX7 was the stand-out when using single coils through vintage Fender amps. The NOS 12AX7 tubes compressed more and sounded "sweeter" and laid back, but the Tung-Sol made the Z2 very "present", very articulate and the lower strings had way more definition. Tjhe gain structure is excellent with the Tung-Sol. I was expecting the lower gain tubes (AT7, AY7) to be the shoe-in for me, but they all made the Z2 more "receding" and too "smokey". Not my thing at all. So, if you have a Z2 and have been wondering if it's worth 30 minutes of your time to sxplore tubes, I'd say "Yes!". You may end up liking the stock tube best, or you may like the laid-back sound/feel of a NOS tube. But if you have a new Tung-Sol sitting around, do yourself a favor and give it a try. The Z2 really has improved "bark" and is very toneful with that tube. In some ways, it's closer to the original Zendrive. The funny thing is that I never liked the Tung-Sol as a pre-amp tube in any of my Fender amps, that's why I had one available. Now it's got a new mission!