I think the culprit is the in-wall splitter but rather than filtering out analog cable ch 6 I am inclined to think that it gets cancelled out as a result of intermodulation between carriers. Do not ask me which ones because I do not know. But if indeed the splitter is filtering out ch 6 on 1 leg then that most be the perfect filter ever since you say that quality on adjacent channels (and also any other) is perfect. A real filter of any kind or technology will always degrade signals of adjacent channels to some degree, only an "ideal" filter in theory does not alter adjacent channels (although there are very good filters out there that do an almost perfect job).

I have a similar problem in my home, for some reason (I beleive it started after the last firmware) my 9242 PVR from Bell TV only outputs TV2 tuner on ch 85 no matter which ch I select on the modulator menu. Guess what?, if connected straight to the TV I get perfect picture on ch 85 but throughout the in-home cabling nothing , only snow , not even an out of sync weak signal (kind of horizontal lines) and the funny thing is that ch 85 on Rogers has nothing there , not even digital carriers (unless the internet service is there) because my QAM tuner does not pick up anything not even a scrambled QAM channel on 85. It has to be some sort of cancellation due to intermodulation since even CH 33 UHF coming from Buffallo is too weak (or maybe CH 32 WNLO 10 Mhz below could be the culprit). Cable ch 85 has a similar frecuency as OTA UHF ch 33. I know there is a lot of ingress from the powerfull OTA signals on the in-home cabling since even cable ch 95,96,97 that use the FM band get a lot of interference and of course the typical Ch 5 and Ch 9 that you can even see CBC and CTV in the background caused more likely due to a poor ground. I can even get CBLFT ch 36 UHF with lots of snow on cable ch 76 (after I installed a ch 72-78 to create space for up to 3 new modulated tv channels from my satellite receivers to distribute them thoruout the house). I definetely have to properly ground the whole system (I think it is the 3x4 FTA satellite switch outside).

Funny how a particular signal or channel might affect others eh?.