Sounds like either a video opamp has failed or the power supply within the unit it faulty.
Does if work OK is the IR upgrade is unplugged ?
Check the voltages as follows, put the multimeter negative lead on a ground point, the tab of U901 is the most convienient place.
on the metal tab of U903 you should have around -10 volts or slightly more. If this is a lot lower or non existant then U902 the voltage inverter has probably died. ICL7660A As the audio is also distorted my guess is a power supply fault.
If that is OK, check the voltages on pins 8 and 4 of the opamps. Pin 8 should be +5v, Pin 4 should be -5V
There should be no noticable DC offset on the outputs of the opamps, pins 1 and 7, they should normally be under 20mV. If there is a large offset voltage then the opamp is probably faulty. The video opamps are AD828's and the audio opamps are NE5532's
All ICs are socketed so swapping them over is easy.
If you want to send it back I will take a look at it for you.
Dont give up on Tivo just yet, work is underway on an alternative EPG and I am hopeful that my spare unsubbed Tivo will be given a new lease of life as well