Maybe I'm misunderstanding about the cable box thing...
I thought that your problem was that the output was always channel 3, but you were trying to tune with the cable box or VCR or whatever. If you use and IR blaster controlled by LinuxMCE, you could have the IR blaster change stations on the box you are trying to send output to your analog card from. Every station, as far as the source/channels.conf is concerned, could be set to channel 3 for the WinTV-150 inside MythTV,... That way,... You would send IR signals to the cable box to switch to,... say channel 21,... but the MythTV setup would pull that in at channel 3... Don't know if that'd work,... but worth a try...
Personally, I had the Haupaugue (sp?) HVR-1600 (2 of them, actually) and I pulled them both, because they were such a pain in the backside to configure. I rely on the four tuners in my 2 HDhomeruns.
It seems like your indication is that the tuners aren't pulling in a signal...
As far as the tuners are concerned, you might need to boost your coax signal a little bit,... especially if it's also your source of broadband internet. Multiple splits can degrade signals, causing one or both tuners to fail to lock in signals (a worse problem with QAM signals than analog). I used an amplifying RadioShack coax booster that's claimed to be digital/broadband compatible. If your cable is your broadband, you might want to make sure that your cable modem is working off the first, non-boosted split, and boost the cable TV signals off the second branch of the split.
There are sometimes signal strength issues with cable Co.s,... monitoring your signal power levels and adjusting them very fine to; (a) prevent multiple unbilled splits & (b) cause you to wrack up service charges on your setups. Their fine tuned settings in their equipment can mess with your internet connection.
Another thing to look at is the Mhz rating on the splitters and cable being used. You want to make sure that it's high quality enough for digital signals,... the cables preferably being well shielded. There are some shielding/signal improving tricks you can use, as well, when you have a lot of different cables (power & data) running close together and are fearing signal interference (like using a grounded foil wrap to provide extra shielding, or using a ferrite ring or bead on self-wired data connections).
PS: Regarding output switching... I have an AV switcher that automatically switches to the live connection from a number of different outputs,... don't remember where I got it, but probably RadioShack. No IR involved. The main thing is to turn off output from the device you aren't using... Causes a hassle when my stepson forgets to turn off the Wii, and the Mrs. wants to switch the TV off LinuxMCE over to regular cable. For the technically gifted, you could also turn on/off devices with Z-wave, serial or something, as defined by a LinuxMCE viewing scenario... An auto-switching AV box would then just send the "right" signal to the viewing/listening device (TV and/or Amp).