Akai 747 w/DBX question

I would take a pass on the dbx unit. If you really feel you need noise reduction would look at one of the pro units, dolby A 363, or a Dolby SR unit.

Other three options would be
1. get a set of half track heads for your 747
2. install a Vishay foil resistor at the loading position on the PCB where the signal from the head first goes to the board.
3. replace the wire from the head to the PCB with Mogami Console wire.

items 1-3 above will each reduce (increase S/N ratio) by a few db at 7.5 ips tape speed. 

I would suggest demag'ing the heads every so often. Otherwise you will lose high freq response. Just make sure you use a demager with a protective tip and move the demager SLOWLY, you want remove magnetic fields from the heads not add them.

I demag the heads on my decks before doing any serious recording.

The stock load resistors on the 747 are carbon film. you may think the deck is quiet now at 7.5 ips, but wait til you listen to it with Vishays, you'll think you got 30% more musical detail coming through.

The other big benefit the 747 deserves is bypassing the audio stage coupling caps with film and foil caps.

Incidently the Vishays do wonders for MM and MC cart loading positions, but not trying to hi jack the thread.

I would demag the entire tape path. The key in demag'ing is to move the unit sloooowwwwly. If you move it fast across the heads very little demag will happen and you can actually increase the mag level on the heads.

it will vibrate a little since the magnetic flux field is interacting with the metal parts on the head or rollers.

and as one other poster noted, do not use a bulk tape eraser.