what to buy first

I have a marantz sr 7011 and bookshelf B&W 685.  I am using a benchmark dec running off my mac mini.  I don't have plans on getting a turntable.  I would like to do tubes amp and preamp.  I have an extremely large open room with 30 foot high ceilings and lots of glass.  Window treatments are not an option.  I have one area rug, but mostly tile floor in the room.  I have read about the monitor platinum 5 speakers and on paper they sound like a nice set, although I have not heard them. I will use the receiver and speakers in another room eventually but I plan on using the dec and mac mini on my new system.  Thanks for above responses.

Interesting question.  The Benchmark DAC/Mac-mini is a well-respected combination for a source.  However, based on my experience with a  Marantz SR5006 AVR, that is probably your weak link. Despite the power rating, it sounds very anemic to my ears.  I would go for as good an integrated amp as you can afford right now.  There are quite a few tube integrated amps advertised right now on A'gon across a very wide price range.  If you give us a budget, we can make some suggestions.  I agree that you should stretch as far as you can to get a good foundation and also agree that if you go for separates, then a really good pre-amp is the heart of a good system. 

For those who say start with speakers, I would reply that the Marantz AVR will not give you even a taste of what really good speakers can deliver.  Speakers that are designed with tube amps in mind are going to be very disappointing when driven with that Marantz.   And if your goal is a tube amplifier, then matching "tube friendly" speakers to the amp is critical, once you upgrade your speakers. If your budget can stand it, a decent sub-woofer with the tube amp driving the B&Ws might be a good idea.  And consider replacing the jumpers on the back of the B&Ws.  Cardas make some reasonably priced copper jumpers. 

Is there a component you desire and have been wishing/lusting for?

I'd start there.

mhsmith What should be purchased first?

Speakers first pick the ones you like the sound of first in good Audio shops, as most shops will have them driven an amp that does them justice, as speakers can be hard or easy to drive.

Once you’ve got the speakers, come back here and ask which amp/s will drive them best, as some of here will look at the specs impedance/phase/loading etc graphs of the speakers you’ve purchased and recommend the right type of amp/s

Cheers George

While different makes and models of speakers clearly sound quite different and some measure out better than others in instrumented tests, they can only work off the signal fed to them. 

The SR 7011 is a decent AVR, so it's likely that better speakers may give you sound quality that you like and enjoy better, i.e., more and deeper bass extension and possibly better tweeters.  But i also agree with swampwalker above that amplifier quality can be just as important.  And in my experience cables can also make a big difference, although many swear that is "fairy dust". 

I am currently running a Denon AVR-X5200, which is pretty high in the Denon lineup, but using a separate class A power amp, which is a Krell FPB-400cx.  I do believe the Krell improves the quality of the sound over the built in amp in the receiver.  If you know what amp you want to end up with, I guess you could use it with your receiver and see how it compares.