recording to Reel to Reel directly from "preamp out"

I'm assuming an integrated amp with pre-out also has power-in. Not necessarily but often times the way it is.  

Headphone monitor will be better as that will allow volume adjustment and not require an interconnect.

Yes of course you can. Just remember preamp out is variable output, the volume control will affect your signal level on the deck. You can even monitor, simply connect the monitor out from the tape deck to your power amp. Just remember to not touch the volume control while doing this or you will mess up your tape level.  

I would play around to find the preamp volume setting that will allow you to adjust the tape record level using the tape deck level with the tape control somewhere around the middle of its range. Not critical but you will probably find it easier to get a good level this way. Then just remember if you do try and monitor using the power amp the signal coming off the tape deck is driving the power amp. So it is what it is in terms of volume, you will not be able to adjust it without messing with your record level.