I just received a ZROCK3 and added it to my system (Rega Elicit Mk 5, Custom modified by Magnastand, a pair of Magnepan MG-12SQ speakers, a Linn LP12 with Sound-Smith Carmen phono cart,).

With the tube that was supplied by Steve, the bass was a little overwhelming. The knob on the front panel could not go past the one o'clock position.

The solution was to try tube rolling.

A 1957 Raytheon 12AU7 PLATINUM PLUS Grade Preamp Tube 90% NOS found for $21.95 on eBay, solved the issue (tested at as NOS).

Now with a less bass-heavy 12AU7 tube, I can move the knob to the past two o'clock position. This provides a tighter bass that I can still feel in my body.

Moving the front knob higher allows the ZROCK 3 to tame the high end.