Best investment for upgrade a RB-1090 or MC300

I would try and get everything close to the same level. The 802ds are very revealing speakers and any improvements made will be easily heard. Running with a balanced preamp to amp makes a big difference and might be the cheaper alternative.

My vote would be for a better amp and forget about a receiver for those speakers. Then again when the upgrade budget is available again go for a source. Last should cables. I say that because when I had a Bryston B100sst and I tried different sources and had to 'squint' a little to hear the difference between sources. I bought a Meridian G08. Then I upgraded the Byrston to an Octave V70SE. I just recently I put back my original Yamaha cdx1030 (which I did have to squint a bit). As good as the Yamaha seemed then I can't listen to it now. That has been my experience and why I recommended that order.
Good Luck

Thanks for the great information, has anyone compared the B&W 802d's with the MC352 and MC402? Is the extra cost for the MC402 worth it?

Ditto on the 1090 in regards to balanced operation. However, I do own both the 1090 (for Paradigm Ref 100v4 mains) and a 1095 that runs everything else. While both of these Rotel amps share that "endless/effortless power" quality, have excellent soundstages and a nice neutral frequency response, the 1090 is noticeably smoother in my system and excells at bass control/response and a slightly more refined high-end compared to it's multi-channel brother (when driving the mains). So I would do that as a budget-friendly amplification upgrade, and focus on a better preamp as well. Or get a preamp first, and see where that leads you...