I retubed mine because it had tubes in it which were eight years old so I have no real basis for comparison like some of the other guys do.
With any gear I've had which uses tubes, changing them out tends to have the most effect if the existing tubes were older and in need of replacing. Changing out an existing new tube for a similar model from a different manufacturer can change the sound but it's all fairly subtle from what I can tell. Great stuff if you're talking about recording gear and your genre is jazz, not such a big deal if you're talking about stage gear and your genre is metal. I've never plugged into an amp and thought "this amp clearly has Amperex Bugle Boys in the first two slots and a Telefunken in the third slot" even if I had headphones on. It either rocks or it doesn't.
Ultimately I do think the money is best spent on retubing once every year or two rather than worrying too much about endless subtle comparisons.
As far as tube slots, you will get the most benefit from swapping the tube in V1 as both halves of that tube are dedicated to the initial preamp stuff. Half of the V2 tube is for the Deep switch so if you use that, you may see some benefit there. V3 is dedicated to the effects loop so if you're not using that you may never notice the new tube there. V4 is the oddball tube (12AU7) and drives the power section of the amp.
There is a thread devoted specifically to tube swaps in the DB750 that is worth reading through if you're interested in doing this, though you already have replacement tubes so experimenting with it will serve you far better than reading a web board full of opinions on the topic.