Smorenc,
What a beautiful amplifier! I have the same VTA ST-120 with KT120 tubes, but it is nowhere near as beautiful as yours.
My experience (and with the same amplifier as yours) is that it will definitely sound better plugged directly into the receptacle and bypass the power conditioner. I would also recommend (if you have not done so already), that you upgrade the power receptacles themselves.....that is worth another bump in sound quality. I can highly recommend 3 receptacles: Furutech GTX-D(G) gold plated, Furutech GTX-D(R) rhodium plated, or the Oyaide R1. All are around $200 give or take, and well worth the upgrade....or at least were in my system.