Hi Funky:
You are correct, the 8 ohms rating for both your Marantz and B&W would indicate that they are compatible, at least on paper.
I originally thought that your B&Ws were not rated at 8 ohms and because of the power anomaly with the Marantz, you could be running out of gas ... but that is not the case. My apologies for not doing the research and relying instead on memory before posting for the first time.
So, I was using two different Marantz receivers (2216B and 2240) for a number of years. I used them with a number of different monitors. The pairings I liked best were the following: Rega Ara, NHT SB2, Wharfedale Diamond 9.0, and Omega Super 3i. The pairings I liked least were the B&W 302 and the EPOS ELS3.
Like yourself, I was getting good sound from the Marantz, but I felt that there was more to be had. Even though I grew up with warm, organic sounding stereos (think Philco tube amplifiers, vintage Pioneer receivers, Dual turntables, and EPI and AR speakers), over the years I have become more accustomed to the sound of more accurate, slightly leaner sounding separates and CD players.
So what does it all mean?
Leave well enough alone. I think you can only take vintage Marantz receivers so far. They will only sound so accurate. Your system most likely sounds as good as it will. That's a good thing ... you can stop right here.
If I were to do anything, I would consider changing speakers. I was most excited about how my Marantz receivers sounded with the Omega single driver speakers. Single driver speakers can be unforgiving with badly recorded music though and can sound 'honky' with vocals. Omega has done a lot of work to correct this. I would look at their new line
Hoyt-Bedfords .
Best of luck,
Rich