Is McIntosh 6900 enough for B&W 802D?

Yes. It will make music beautifully with that amp period.
Get the latest 802's and pair it with your old Mac and get ready to listen and listen and listen and.....
When you are ready to move forward you should look at all options for those superb speakers and not just think of the McIntosh line. There are others that make B&Ws simply sound amazing.

Sure I know I can do better. The question is can it play decent sound with 802's until it gets an upgrade or will it sound like crap?

You can do better than the 6900. I owned one, very two dimensional.

Not enough power for optimal performance.

IMO

Personally I don't think so.

When I mean decent I mean not sounding like the amp is dying, I mean clean natural bass, mids and highs. I heard underpowered B&W's (800D in particular) - they sounded like utter garbage so I know what happens when B&W's don't receive enough juice!

Enough to make them work? Certainly.
Enough to get the best out of the 802D? I don't think so.
To sound decent? Well, that means different things to different people. Some here "can't stand" some VERY expensive CD players...I tend to think any of those probably sounds great, but likely some sound better than others...

Can't you audition? B&W and McIntosh tend to be sold at the same dealers.

Nice deal on a Mac 7270 right now. Should be sweet on
some 802D's or Vandy's.