If you just want to test your multi-switch's then hook one up stand alone and see if it works. If it does hook the other one up and see if it works. If they both work seperately than they're fine and it's something else. You have stated what channels are impacted on your HD DVR's. Are they HD or SD? What are the transponders the channels are on. What is the signal strength for the transponder. You're quick to say it's not something without actually giving any information. Without asking information we can't get the information to help. If you don't want to give the information we ask for then I guess you're not really wanting help. It's not like there's a universal "do this" and everything works. According to your actual post you don't even need a second 6x8 because you said you had 4 HD and 3 SD which is only 7 tuners which is fine for 1 6x8.