From what I've read from personal reviews, etc., having a built in HD radio tuner that can't be disabled is sometimes more troublesome as you've found with the signal. Outboard add ons can somewhat remedy this if you can select the outboard HD/standard combo tuner versus the internal standard only tuner which can avoid HD signals.
The Sony MEX-BT3700 has built in BT, Ipod, USB, outboard HD add on option, which doesn't disable the inboard tuner, but you'll have to run an antennae splitter to go to both.
JVC has a few HD built in tuners, but have the signal in and out issue you mentioned and the HD tuner can't be disable as I understand
Sorry this isn't exactly your question, but I figured you're back in the market for a tuner and might re-consider the HD thing. I'm also looking at a new head unit for my truck and am in the same decision process.
Why do all of my threads suck? Is there an internet posting school somewhere? -wes k