In answer to your questions first off I don't think this unit has a stereo tuner. At least it does not sound like it does and the owners manual does not state whether it does or not.
No, this unit does not have chase playback during timer recordings. In fact you can't really do anything else while a timer recording is in progress.
The final question that you asked is can you replace the hard drive like the Polaroid? I tried putting in a 300gb HDD and it took it, but the unit partitioned it at 80gb. So I then hooked up the original HDD to my computer and used Norton Ghost 9.0 to make a backup of this drive. I then created a FAT32 drive on the 300gb and used Norton Ghost to restore the image on this drive. This allowed about 120gb to be used, but I think if I ran a Clean Drive through the units menu this would probably re-partition the drive back to 80gb.
The picture quality is on par with the Polaroid. Probably because this unit also has a LSI encoder chip. I don't notice too much of a difference between XP & SP, but LP is noticably less sharp. I didn't test any bitrates below this. In my experience anything below 4 hour mode on a single layer DVD is not even worth bothering with.
The remote operates a lot better than the Polaroid and does not have the range problem of the Polaroid. In fact the range of the remote is one of the best i've ever come across.
One quirk that I did come across is that if you edit the chapters on a non finalized disc this can cause the burned disc menu to operate weird. Usually it creates a menu with both a thumbnail mode & a text mode, but if you edit chapters on a disc you can't always go to the text mode or the DVD player locks up.
The picture quality of the tuner is a little better than the Polaroid, but my Sony & JVC still looks better (Less noisy).
My conclusion is that this is a better unit than the Polaroid. Especially since Radio Shacks recent price drop to below $200.00.