Can't help with the tube issue - my Marlin 80DL has the box magazine.
For a really good peep sight, I used the "Lyman 57", with the "57MR" base for Marlin rifles. The 80DL and 81DL already have the peepsight screw holes on the left rear of the receiver, for the Marlin peepsight. I like the Lyman design better. I have found them on Ebay for decent prices. This same sight uses different bases for other rifles - 'MS' for Mossberg, 'RS' for Remington, etc.
My Marlin also wears a target globe front sight - kindof a "poor man's target rifle".....
