Those Ref 750s have 16 KT 88s sitting on top of the amps with only air around them, right? You did not say what speakers you have. If you have speakers that have large woofers pointed at your tubes, I think they could affect the sound.
I suggest you move the amps around the speakers, to the sides, behind the speakers, etc. until you find where your system sounds the best, and that may be with the amps in front of the speakers.😁 Use some furniture sliders to move the amps around. We don't want to hear about you ending up in the hospital.☹