Transformer noise can be a problem. I have two 2200 m-blocks and one of them hums when it gets warm but foutunatly it is not bad. My 7500 is dead quiet, knock on wood. One thing that aggravates the problem with the 7500/7700 amps is both transformers are located directly behind the front panel.
Too bad you had that problem because the 7500/7700 are great pieces of equipment.
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Epson 3500 3D projector
93" Carl's Place Flexiwhite DIY screen
Outlaw 976 Pre/Pro
Outlaw 7500 for LCR & L/R surrounds
1- Emotiva Mini X-A100 for rear surrounds
2- Outlaw LFM-1 EX subs
Oppo BDP-105 Blu-ray/DVD/SACD player
Outlaw LCR (Snell Labs) for center channel duty
Tekton Lore monitors L/R
4- Emotiva ERD-1s surround speakers
Monster HTS3600MKII power center