Using Tape Out from Preamp for powered sub woofer.

It's not a stupid question, but you cannot use it for a sub. :)

The problem with the Tape Out loop is that the volume control doesn't control the level so your sub will always play at maximum.

Better to get a Y splitter, or use the speaker level inputs on the sub, if available.

Best,

Erik

Couldn't the OP use tape out and control the volume from the sub as he/she wants different levels depending on volume?

Noromance, your question is correct. 
To clarify my point further, I do have volume control on my powered Sub.  And I need more bass when I'm playing music at lower volume, which is the case most of the time. And as I increase volume from preamp the bass from the sub is too much for my taste. And I hate to standup every time to reduce bass level on the sub. So I want to bypass the preamp volume control for subwoofer. I would like to try this arrangement, but just want to make sure it's technically correct and will not damage preamp or the Sub. 

As long as you don't mind adjusting the volume on the sub, I see no issue. Try it and see starting off at the lowestvsetting on both pre and sub.