Hello,
The reason why turning on channel 3,4,7, or 8 reduces the bandwidth of all channels to 5GHz is because certain channels share the two ADCs within the scope. As more/certain channels are turned on, the bandwidth of the sample rate will decrease because a portion of the ADCs processing power is being used by another channel. Below I explain a breakdown of what channels should be used to receive certain sampling rates and bandwidth.
• With MSO68B channels 1 and 5 ON (2, 3, 4, 6, 7, and 8 off), the scope can sample at 50GS/s and 10GHz.
• With MSO68B channels 1, 2, 5, and 6 ON (3, 4, 7, and 8 off), the scope can sample at 25GS/s and 10GHz.
• With the MSO68B with all channels ON, the scope can sample at 12.5GS/s and 5GHz.
This information can be found in the Performance Verification Manual on Page 22-24: https://www.tek.com/en/manual/oscilloscope/6-series-b-mixed-signal-oscilloscope-specifications-and-performance-verification-6-series-mso
The max sample rate associated with each channel can also be found above the scope channels on the front of the oscilloscope.
I hope this helps clarify the how the scope channels are related.