When we brought the MC452 into our system with the existing CJ Act 2 Series 2, there was a serious impedence mismatch. Because the MC452's input impedence is 22K Ohms, you should consider possible impedence issues with any tube preamp. We then introduced a Pass external crossover (XVR-1) to split the signal between the monitor mains and sub. The Pass XO completely solved the impedence issue and we have been very happy with the Mac/Act pairing (particularly after rolling the ACT2.2 to NOS 6H30 DR). The MC452 is quite neutral and transparent; it will reveal changes up front. Good luck.
